Welcome to the Forum---probably the friendliest of all the forums. If you bring it up to '540', you will NOT need to enlarge the crankcase 'barrel holes'. If you go up to '595' you quite probably WILL need to enlarge the barrel holes (I will measure my spare 595 crankcase for you). I have written a technical paper regarding the tuning of the 500 engine---if you would like a copy, contact me direct on---
[email protected]. Balancing the 500/126 crank is normally not needed---the 'from the factory' balance is quite good (just don't continually exceed 6000rpm!). There is also the problem of finding somebody who CAN balance a '2-cylinder' crank---it is VERY difficult on this side of the 'big pond'---almost impossible on your side. For honing/boring barrels I would suggest that you try a 'Motor-bike' workshop---they will be experienced in dealing with individual barrels.
With regard to fitting bigger valves, for 'road-use-only' you only need to enlarge the inlet valves---I suggest +2mm. There is no need to fit bigger exhaust valves. What DOES help is 'porting' the cylinder head and exhaust 'elbows'----the standard head is not renowned for its 'gas-flow' ability!
The 500 (110) engine is small and very easy to strip/rebuild, but I always reccomend that you mount it on a simple engine rotisserie---makes it much easier to work on.