In an effort to clarify as opposed to contradict Miki's statement, my understanding is that the 587, 657, 657x, and 720 share a common mounting setup. As it has been described to me (through YouTube I believe), you can physically swap between any of those four motors with little in the way of mounting changes. The 787 (800) and 951 are significantly different than the 587-720 and thus, would be very involved to transplant into an older 587-720 machine (as Miki stated).
In order to get an older machine operating correctly and taking advantage of the additional power from an engine transplant, you will need to move the MPEM and impeller (possibly entire pump assembly) from the more powerful setup into the upgraded machine. While a 587 MPEM may run a 720, it will not allow the 720 to produce full power. Additionally, if you put the 85hp of the 720 through a 587's 56hp impeller, you will over rev the engine and not move very fast through the water. I also believe, but am not sure if, the exhaust system is sized to the engine so parts of it would need to be upgraded as well.
My interest in this topic is that I am considering a 657x transplant into my '94 GTS. Yes, I know, the 720 would be a better choice if I wanted the best power and to take advantage of more common performance parts but I already have two 657x engines in and two spare jet pumps for my '95 Speedster. My thinking is that it would make sense to keep all three engines and jet pumps the same.