I need help with making 2 custom fields created to be required if the Do not create Opp checkbox is not checked. If it is checked then the 2 fields shud be required. I also need to map the value of these fields to 2 custom fields in Opportunity. Code from Class below : 
 

public with sharing class ConvertLeadExtn
{
    Final Lead curLead;
    public boolean IsSendEmailChecked{get;set;}
    public String SelectedAccType{get;set;}
    public Contact ContactAcctToRefer{get;set;}
    public boolean IsOppChecked{get;set;}
    public String SelectedConvrtdStatus{get;set;}
    
    //Returns the list of account types
    public list<selectOption> lstAccountType
    {
        get
        {
            List<selectOption> lstAcctType = new List<selectOption>();
            lstAcctType.add(new selectOption('','--None--'));
            lstAcctType.add(new selectOption('Create New Account','Create New Account'));
            lstAcctType.add(new selectOption('Attach to Existing','Attach to Existing'));
            return lstAcctType;
        }
        set;
    }
    
    // Returns the converted Status list
    public List<selectOption> lstConvertedStatusList
    {
        get
        {
            List<selectOption> lstConvertedStatus = new List<selectOption>();
            LeadStatus leadStatus = [Select Id, MasterLabel from LeadStatus where IsConverted=true limit 1];
            lstConvertedStatus.add(new selectOption(leadStatus.MasterLabel,leadStatus.MasterLabel));
            return lstConvertedStatus;
        }
        set;
    }
     
    //Controller
    public ConvertLeadExtn(ApexPages.StandardController controller)
    {
        this.curLead = (Lead)controller.getRecord();
        if(curLead.Id!=null)
        {
            ContactAcctToRefer = new Contact();
        }
    }
           
    //Converts the Lead
    public PageReference ConvertLead()
    {
        Id accountId;
        PageReference accPage;
        try
        {
            Database.LeadConvert leadCnvrt = new database.LeadConvert();
            if(SelectedAccType == null || SelectedAccType =='')
            {
                ApexPages.addmessage(new ApexPages.message(ApexPages.severity.ERROR,'Please Select Account Type!'));
                return null;
            }
            
            //Sets all the fields that are required
            leadCnvrt.setLeadId(curLead.Id);
            leadCnvrt.setOwnerId(curLead.OwnerId);
            leadCnvrt.setSendNotificationEmail(IsSendEmailChecked);            
            leadCnvrt.setDoNotCreateOpportunity(IsOppChecked);
            leadCnvrt.setConvertedStatus(SelectedConvrtdStatus);
           
            if(SelectedAccType == 'Attach to Existing')
            {
                if(ContactAcctToRefer.accountId !=null)
                {
                    leadCnvrt.setAccountId(ContactAcctToRefer.accountId);
                }
                else
                {
                    ApexPages.addmessage(new ApexPages.message(ApexPages.severity.ERROR,'Please Select Existing account to attach!'));
                    return null;
                }
            }
            
            if(!leadCnvrt.isDoNotCreateOpportunity())
            {
                leadCnvrt.setOpportunityName(curLead.Company);
            }
            
            Database.LeadConvertResult leadCnvrtResult = Database.convertLead(leadCnvrt);
            accountId=leadCnvrtResult.getAccountId();
            System.assert(leadCnvrtResult.isSuccess());
        }
        catch(exception excptn)
        {
            String excptnMsg = excptn.getMessage();
                     
            if(excptnMsg.contains('CANNOT_UPDATE_CONVERTED_LEAD') || excptnMsg.contains('INVALID_CROSS_REFERENCE_KEY'))
            {
                ApexPages.addmessage(new ApexPages.message(ApexPages.severity.ERROR,'Lead Conversion Failed! Lead is already converted!'));
            }
            else if(excptnMsg.contains('TRANSFER_REQUIRES_READ'))
            {
                ApexPages.addmessage(new ApexPages.message(ApexPages.severity.ERROR,'Opportunity can not be created, User do not have Read Permission!'));
            }
            else
            {
                ApexPages.addmessage(new ApexPages.message(ApexPages.severity.ERROR,excptn.getMessage()));
            }
        }
        if(accountId!=null)
        {
            accPage = new pageReference('/'+accountId);
        }
        return accPage;
    }
}