Wildbill went thru similar a couple of years back & I think he had advice from Dale Lineaweaver & others who seemed to know. Try PMing them.
I'm under the impression that so long as both engines are either non e-start or both are e-start they should fit.
If it is any help, there is a bloke parting out a '95 non e-start 501 on German eBay at the moment. He says he is willing to sell the engine as a whole or split it up as required.