I was wondering if one engine has way more miles on it than the other? I would run the low miler . Have you taken a compression test on both engines yet? I would adjust the timing chains on both before anything gets taken apart, dump the oil , drop the pan and sift through the oil for chunks or other wear, then shine a flashlight up to the timing chain and see if it is slack ( and check for wear marks there) , then rebuild( at least) the top end on the high miler if no wear was found at the timing chain area on the lower end and keep that engine as the spare should the other one go south.Yes to swapping the wiring to the the 550 harness, the 500 is more primitive but everything is color coded, just plug and play, unless your 500 has the pin type of harness, then the only swap over that would be needed on the 500 engine if you are going to run that would be the generating system ,include that in the swap from the 550 if that's the case. The 500 transmission is known to be more temperamental than the 550 transmission , and has clutch rods that break once in a while and the adjuster in the lower case cover could have wear from lack of grease or stiff from old grease. Be sure to go through the gears on a center stand to be sure the shifting forks engage and are not frozen before you choose, shifting forks that don't work isn't a good thing to find out about after you make the swap.....means splitting the cases to repair. You must choose wisely Grasshopper.
