In Canada such changes are possible, and are in the thousands to ten's of thousands of dollars for the paperwork. This is the nature of the work I do. I recently completed an STC program to permit the modification of TCM IO-520's to be carburetted, and installed into all older Cessna 180's and 182's.
EASA's and ICAO's far reaching influence however, is making this kind of modification more difficult even over here, as we are now forced to comply with ICAO noise limits, and the testing is very expensive. (yes the IO-520 on the front of a 180 makes no more noise than it makes on the front of the 185 where it came from, but we're still being told to test!)
Bottom line, save your money,and buy the plane you want, the way it left the factory, there is still a great selection out there...
Cheers,
Pilot DAR