wondering what if any are needed 1989 35 cafe racer 420s with trs drives want to install 525efis want to keep trs drives

Bell housings, couplers, swap ring gear to the back of the flywheel, and you'll need to either redrill the heads for the exhaust off the 420's or get custom tails.

Also the engine hatch I'm pretty sure won't fit And need to be modified for extra clearance. Because with the 525's I'm sure you'll want to run the mercury headers that come on the 525 EFI's which would hit the hatch. But the 420 exhaust could work, but that would hinder the performance correct?

the 420 exhaust would live longer than 525 headers...

Sure, but if your dropping the coin on the EFI's I'm sure everyone would want the fancy smanchyyy shiny big tubes from the merc exhaust. At least one would think

I'm not sure there would be a clearance issue. I had big tube CMIs on my 750s and they were even height with the flame arrestor. Single plane intake with a dominator carb.

I had an 89 35 with 500s and the tall Merc Gill tail pipes with no clearance problems.

Same tooth count. And Phil do a search about 525 headers. Another post proving your ignorance.
There is plenty of hatch clearance also.

You can't put the Gil on a 525 the bolt pattern is incorrect. There is no clearance issue with the 525 . Jamie pretty much stated what's needed. If it a std 160 tooth ring gear the starter won't be a problem. I don't know of it's a differant tooth count. Doubt it but just a thought u had,

I believe that the cannon plug is different on the old 420 harness compared to the 525efi.