I have a 29 Fountain Fever with twin HP502's, yesterday the port motor spun a rod bearing. Don't know the extent of the damage yet, but my question is if the crank turns out to be pretty bad shape can I put a 540 rotating assembly in it and run it the rest of the season with the other engine being a 502?

Yes. Won't be optimal at all but it won't hurt anything either. I wouldn't try any top speed runs, just cruise/putt around.

A 29 fountain with twin 540's will be bad ass. So buy the one then put around this season.This winter buy the second one and sell the 502

No, won't work. Port motor will be too strong and will push boat into starboard turns all day. Sorta like NASCAR but opposite direction.

Same RPM with the same prop should be the same speed on both engines no?

What's the HP difference gonna be? I doubt you will notice much difference if all you are changing is the CID and the valve train/cam/heads. One engine will prob be just a bit lazy compared to the other. I had mismatched cams in a set of engines once and only after tear down and inspection was when I realized why the one engine was lazy.

I agree. The two props will be turning at the same speed so it won't make a hoot of difference when you're running.

Getting her up on the plane, I imagine the new engine will do the job more easily so a little less on the throttle shift to keep your revs in line. I think you should run her at the capability of the lesser engine and not the other way round, don't want to damage another engine by asking it to keep up with the more powerful one.

RR