Why is it that fettling with bikes usually opens up a large can of worms?

I decided to replace my cam chain, tensioner, and guides as it was getting a bit noisy. So setting aside a few days to get on with it i got out the spanners.

Everything went fine, engine out of frame on work bench, rocker cover off, i started to undo the cylinder head bolts and noticed one very under torqued compared to the others!Sure enough when it came out i noticed that tell tale sign of aluminium in the bolt thread arrgh!!!

Resolved to the fact that it was going to have to be fixed, i unbolted the barrel. Inspecting the bore and piston, i could see it was time for a rebore. time for horror number two...

On measuring the bore and piston i noticed it wasn't anwhere near the 100mm for the Xl but nearer 97.8 of the XR600.

So now i realise that the previous owner has replace the standard XL barrel, piston, and head (cam-shaft is XR spec to)with that from a XR! Not only that but when i cleaned the piston i noticed it stamped with 1.00mm! so it has already been rebored a couple of times!

...So my question after al this ramble is...

is the barrel at it's rebore limit (it has a steel liner not coated ali)?
Or should i get it re-sleeved back to original XR spec?

Is there a good and trusted company in or around London to get this done (or in the UK)?

Are there problems waiting to happen with any of the other components because of this XL/XR mix?

Cheers

Stuart (off the road)