The driveline coupler is 2 halves like this:
And there's a rubber 'spider' that goes between them,
You will most likely need to cut new threads into the coupler, or possibly on your driveshaft, for it to work. You may also have to cut your driveshaft shorter if you go with a 4 stroke, as it's a few inches longer than a 2 stroke 3 cylinder engine.
If you are lucky and the threads of your driveshaft are larger than the existing internal threads of the coupler, all you have to do is put the coupler on a lathe, bore it out, and re-thread it to match the threads on your driveshaft.
If, however, your threads are much smaller, then you will need to bore out the coupler to 1 inch diameter, and TiG weld a piece of 1 inch solid round to the coupler. (As long as you bevel the joint and weld it properly on the front and back sides, it will hold, trust me!) After welding the solid round stock in place, put the coupler on the lathe and proceed to bore and then cut internal threads to match your driveshaft's threads.
Drilling/boring/threading the coupler is a precise operation....the threads must be cut square, and the hole must be centered. Any mis-alignment will cause the coupler to run eccentric, and this can lead to failure of thru-hull bearings. In other words, a lathe must be used. To simplify the operation of cutting the internal threads, you can use a tap, and secure it in the chuck of the tailstock, and unlock the tailstock lever, so it can move along the bed of the lathe.
MAKE SURE THE LATHE IS TURNED OFF, AND PLACED IN NEUTRAL FOR THIS NEXT OPERATION!! Start the tap into the hole of the coupler by sliding the tailstock toward the chuck until the tap contacts the coupler. Begin turning the chuck of the lathe by hand, while maintaining slight pressure with your other hand against the tailstock. After a couple of rotations, the tap should begin to bite into the material. Once you get it started, proceed to cut the threads in this manner, using a wrench or other appropriate tool to turn the lathe's chuck, and remembering to back it out and use some compressed air to blow clean the shavings out the hole every couple of turns.
Yes, it's a slow, PITA process to cut those threads. But unless you are good at setting up the lathe to cut threads, and you have a good sharp bit made for aluminum, I don't recommend trying to cut them by conventional means. And under no circumstances should you apply power and try to cut with a tap....you will shatter the tap like glass!!
Engine/pump alignment is as follows: when you set the engine on the motor mounts, and slide it back to engage the 2 halves of the coupler, you will need to determine if it needs to be shimmed. Using a small straight edge, lay it across the 2 halves of the coupler, and, using a feeler gauge, check the gap between the edge and the coupler, on both halves. Anything under .020" is within acceptable margins. More than that, and you will need to use shims between the engine and the mounts to get proper alignment.
Well, that's a general rundown of how to modify the driveshaft/coupler, and install it to match the pump. Hope this helps!