Mike,

Would look at cost to totally rebuild vs purchasing a comparable size machine.  Suggest consulting with your tax expert to determine the financial implications of each based on your present circumstances and projected operations for the next few years.  

If you really want to stay with a machine that will match the capacity of a 20 ft head it might be a good time to get a newer combine.  The way manufacturers keep upping the capacity of machines a 20 ft head machine will be history soon (if not already) and good used ones will be a thing of the past. 

By the way, how many hours do you have on your 2166?    

Just curious.   How many hours does your 2166 have on it?