[Aquadeziac]

Try calling Bills Small Engine Repair in Gilmanton. He is a SeaDoo certified mechanic. He rebuilt the carbs on my GTX basket case that I actually put an SBT engine in. He is tops on SeaDoos in my book and has worked on friends' machines too. And as for SBT, I have nothing but good to say about them. They ship super fast and include instructions with photos for swapping the old engine out and which parts need to be transferred to the new motor. They also have a toll free tech line. You could probably arrange for Bill to do the actual swap and purchase the motor and have it dropp shipped directly to him.
I opted for the Premium Engine which has a full 2 yr unlimited warranty. They even tell you in writing that even if you run it out of oil, submerge it, torch it off...anything, they will replace it free. I am on my second summer with absolutely no problems.
If you have the 951 engine it was most likely the counterbalance shaft that seized and would bet it took out the lower side of the case. That particular engine has a sealed oil reservior for the counterbalance shaft/gears. It has to be checked and topped off regularly with 30W oil. It is however, burried in a hard to get at place and you need a mirror to get to it, therefore it is often forgotten during routine maintainance.(Its about a 20 minute job to do it).
Hope this helps. Good Luck!