I read through your thread about the issues you have with the bike and without meaning to take sides here are my thoughts.
First a cut down subframe is a pretty standard modification for this generation GSXR. I probably still have a couple laying around if you need a spare.
I don't see what kind of shock is on the bike but if it's stock people will practically give them to you to take them off their hands. You can buy one for $15 or $20 on ebay or I probably have at least one that I will give to you just to get rid of it.
If it's a Penske or Ohlins then you have a really nice rebuild-able shock and you should have it serviced just as a matter of purchasing a used bike. Same with the forks.
Change all the fluids when you buy a bike. Water, oil, and brake fluid.
I don't know if the original seller believed that the bike has an engine problem or not, but having owned several GSXR's and having blown up a few engines along the way I view them as a disposable part. We abuse the crap out of them and they break. I had an 01 GSXR 600 and it was on it's fourth engine when I parted it out.
If the engine is gone that sucks and the seller should have disclosed to you that there might be a problem if he thought there was one, but I've been right where you are now more than once but that's the risk we take for buying used. These engines can be had for under $500 and I should know because I've sold a ton of them (actually several tons of them
). I just view it as part of the cost of setting up a new bike. The bottom line is that there are always issues with a used race bike, but overall these bikes are a good deal even with the problems that they may have.
I know Rich has an engine that will fit that bike. It's a 600 but it will bolt right in just as easily as a 750 will. In fact Rich has a shock too. Hmmm...