to put it in youll need to at least loosen the bottomend/case and install the seal, the case halfs have a coating of yamabond on them to seal out airleaks, but this isn't too bad of a job if you put the engine on a bench and do it. The crankseals have a lip around the outside of them that fits in a groove in the case, and the case exerts pressure on the outside to have a air tight seal, to get it in without loosening up the case crush youd destroy it and at least damage it.
you can take a small drill bit and drill a couple holes in the motor mounts to align when you replace the engine if you don't have the offset and center to center tool, drain coolant, unhook lines and set it on the bench. Youll get a repair the right way that will last and not cause you more grief later.