I do not have code for the original Clone button; however, below is code that works for me using a custom clone button to call it.  In addition, this code clones the related Opportunity Partner Record too.  It is an all or nothing process.  Meaning if an issue occurs cloning the opportunity or the opportunity partner(s) then all is rolled back.

The code for the custom button is below too.

In the select query just add all the fields that you want modify the data.  If you do not want to clone the partner record(s) too just remove that portion of the code.

// This class clones an opportunity and all related partner records
public class OppCloneWithPartnerRecsController {
 
    //added an instance varaible for the standard controller
    private ApexPages.StandardController controller {get; set;}
     // add the instance for the variables being passed by id on the url
    private Opportunity opp {get;set;}
 
    // initialize the controller
    public OppCloneWithPartnerRecsController(ApexPages.StandardController controller) { 
        //initialize the stanrdard controller
        this.controller = controller;
        // load the current record
        opp = (Opportunity)controller.getRecord();       
    }
 
    // method called from the VF's action attribute to clone the partners    
    public PageReference oppCloneWithPartners() { 
         // setup the save point for rollback
         Savepoint sp = Database.setSavepoint();
         Opportunity newOpp;
         
         String fmtDate;
         DateTime dt;         

         try {                      
              //Copy the opportunity - ONLY INCLUDE THE FIELDS YOU WANT TO CLONE              
              //WHEN MOVING TO PRODUCTION ADD THESE FIELDS
              // Strenghts__c, Underwriting__c,        

                    opp = [Select o.AccountId, o.Account.Name, 
                                  o.Amount, o.CloseDate, o.Effective_Date__c,                                                          
                                  o.ExpectedRevenue, o.Fiscal, o.FiscalQuarter, o.FiscalYear, 
                                  o.HasOpportunityLineItem, o.Id, o.IsClosed, o.IsDeleted, 
                                  o.IsPrivate, o.IsWon, o.LeadSource, o.Name, o.StageName                                  
                           from Opportunity o
                           where o.Id = :opp.id];


             opp.CloseDate = opp.CloseDate + 365;
             opp.Effective_Date__c = opp.Effective_Date__c + 365;
             
             //format the date for appending to the opp name
             dt = DateTime.newInstance(opp.Effective_Date__c.year(),
                                       opp.Effective_Date__c.month(), 1, 0, 0, 0);            
             fmtDate = dt.format('MM/yy'); 
            
             opp.Name =  opp.Account.Name + ' ' + fmtDate;
             opp.StageName = 'Proposal';
             opp.Products_Sold__c = null;            

             newOpp = opp.clone(false);
             insert newOpp;

             // Copy over the partners to clone
             // Behind the SFDC scenes they actually create the second record on the Partner object
             // and they create the records on the OpportunityPartner object when an insert is executed
             // on the Partner object from their Opportunity page.  
             // Notice the code reads the SFDC OpportunityPartner object and insert into the SFDC Partner
             // object.  If you use the Apex Explorer you will see both objects reflect the same records.  
             // SFDC must have triggers behind the scenes handling this action.             
             List<Partner> insertPartners = new List<Partner>();
             List<OpportunityPartner> partners = 
                     new List<OpportunityPartner>([Select IsPrimary,AccountToId,OpportunityId, Role 
                                                   From OpportunityPartner 
                                                   where OpportunityId = :opp.id and 
                                                         AccountToId <>:newOpp.AccountId]);            
            for(Integer i=0;i<partners.size();i++)
            {
                Partner newPartner = new Partner();
                newPartner.AccountToId = partners.get(i).AccountToId;
                newPartner.OpportunityId= newOpp.Id;
                newPartner.IsPrimary=partners.get(i).IsPrimary;
                newPartner.Role=partners.get(i).Role;
                insertPartners.add(newPartner);                
            } 
            insert insertPartners; 
                       
         } catch (Exception e){
             // roll everything back in case of errors
            Database.rollback(sp);
            ApexPages.addMessages(e);
            return null;
         }
 
         return new PageReference('/'+newOpp.id);
         //return new PageReference('/'+newOpp.id+'/e?retURL=%2F'+newOpp.id);
    }
    
}

Custom Clone Button Code with Confirmation:

if( confirm("Sure you want to clone the opportunity an related partner record(s)?") ){
 window.top.location = "/apex/OppCloneWithPartnerRecs?id={!Opportunity.Id}";
}

 The button Display type should be 'Detail Page Button' and Behavior should be 'Execute JavaScript'.  I gave the button label the name 'Clone with Opportunity Partners' but you could call it what you want.

Once all this is created add the button to the opportunity page.

Hope this helps.  It was from with in these walls that I was able to get this code to work.  What a great bunch of people.