Mate - the work involved in converting a GT750 to chain drive is extreme. It's not something that you could do in your garage, and you won't find much on the net about modifying GT750's - because its not something that anyone does. They were a good reliable mid-range touring bike in stock condition - but they are *not* a good basis for a cafe-racer. Main problem with this model is that not many of the cycle parts are common to the normal chain-drive z650/z750's from back in the late 70's / early 80's. So 2nd-hand parts, when you can find them, are expensive. Brand-new OEM stuff will send you broke inside 5 minutes, if it is still available from Kawasaki ....
The best advice I could give you, is to fix it up - sounds like it may just need a new cam-chain and cam-chain tensioner - then sell it, and use the funds to knock up a cafe-racer using something like Simon's Z750 as a basis.
Here is a picture of a z650 cafe-racer that one of the boys down here in Canberra has recently knocked up - this bike is an absolute corker, and is how a good cafe racer should look.

