One went on Ebay earlier this week, was a 2008 with about 6k miles if I remember correctly, was a buy it now for £3500 or best offer. Is that the sort of thing or are you hoping to find one for less than that ?
I live about 30 miles from Northampton which is where the ad said the bike was located, happy to have a look at it for you if you like. I am no mechanic but would be able to give some feedback. @samieb had one for sale on here recently. More expensive but with lots of extras and it looked in excellent condition. Have you tried contacting him ?
So I asked, he hasn't done one but wondered if the casings were large enough for the sleeve and pistons. Unlike a 750 into a 600, most I think seem ti just drop a bigger motor in, or big bore (is that the 944) Mr R may know more but can't seem to tag him..maybe he's left?!
Stroke is different iirc. Would need crank to. Don't think it'd go in the 800 cases. Wet clutch, different primaries etc. 1000 is a twin plug motor so coils different to. Be easier and probably cheaper to pick up a motor, swap it and recoup some of the cost with selling the 800.