OK, I'll try to keep the story as brief as possible! I swapped out the original engine with a low mileage one from ebay. it was a similar price to the full service it needed and only had 17k on it. I had intended to slowly do the service work on it but I had no garage at the time. At 85k on the bike's clock, engine 2 started sounding a bit "diesely" so, since I now had a garage, I started to strip and rebuild the original. This was a really slow process and eventually the noise of engine 2 got worse until I spotted another ebay engine that came from a crashed bike (it hit a stag on a dual carriageway aparently) and I was the only bidder. turns out it had a bad crank too
So after all that the bike has engine 3 in it, roughly 90k on the clock (I'll confirm this at the weekend) and still gets me to work 130 miles a week.
I'm hoping for £500 for the bike, £100 for each engine, £750 for everything - swingarm, carbs, I'll dig out all the bits and list them when I check the bike's mileage.