I did the exact thing with my Cafe Racer 3 years ago. Biggest mistake i ever made.
I traded my 420s for a pair of 540 tall deck motors (not 502s) I got 5k total on the trade in and paid an additional 10k for the 540s. I ended up having to buy new intakes, carbs and exhaust for the 540s for another 10k along with other things like ignition, etc.
The motors NEVER ran right and I went through valve trains like people go through oil. lifters getting eaten cams getting wiped out...in the end, at the final teardown, the blocks were found to be so old and pitted that the water passages were leaking into the oil.
I got my initial 10k back from the builder but did not get my old motors back, so I ended up being out a set of motors.
On the flip side...my supposed low hour 420s ended up having several sleeved cylinders as reported by the engine builder that took them in on trade....
a real fiasco that finally ended this year when i ripped everything out and dropped in a pair of NEW 496HOS with Bravo XR drives...all NEW from pully to propshaft...
Big thanks to Falcon Hi-Performance for getting be back on the water.