Re: Honda rancher
A 350 motor might swap in, but you'd need all the electrical crap too. As cheap as 350's are you'd be better off buying a 350 and selling the 400.
I've seen Pintos still running too, but as a design, they were still a POS. There's a reason 400AT's are cheap when you see them for sale; nobody wants them.
The 400AT's are iffy enough if you keep them stock. If you mod them, they will not last. There may be exceptions to that rule, but I wouldn't own one. I almost bought my uncle's 06 that had about 300 miles on it. I'm glad I didn't. They guy who bought it has it in the shop all the time. Every one I've seen that wasn't bone stock with <500 miles on it has had blown clutches or a dead Hondamatic. The 400 isn't enough motor to run the Hondamatic.
If you are looking for more power, sell it and buy something different. Sinking any money into a 400AT is throwing away good money after bad.
I was looking for an automatic Rancher for the wife, passed on the 400AT due to all the problems they have, and bought a 420AT. Put a 500 top end on it and an 18% gear reduction in it. Both mods used stock Honda parts from other machines. You get disc brakes, a serviceable transmission, and IRS.