be patient they'll turn up soon enough. try phoning around the local bike repair shops and asking if they have any scrappers with good engines . try your local bike dismantlers there must be some. All of them will ship and their stuff is bound to be warranteed in some shape or form.
If it were me i'd look around my locale for other xs650 owners . Most of them will either have spares or know where spares can be had at reasonable prices.
You really only need the engine (jugs pistons head with cam and tappets etc but if you have to buy a long block then you can break it up and sell the spare bits on here or ebay to offset the cost.
You're gonna need to get some self starting going here if you are going to get this solved.
don't waste your time and money getting your existing engine refurbished its completey and utterly and totally fubar'd just buy a good used one put some new standard rings in, hone the bores , grind the valves , renew cam chain tensioner guides etc and seals and you'll have another 20 years use out of it for little outlay.
If your existing engine hadn't of failed due to detonation you would probably have used it without problem for several years even in the worn condition that its in
When you can afford it get a 750cc kit and sell your old block