Hi - 89mm is about the limit for unless you want to start to get experimental with liners - at which case as David suggests - you'd best have some blocks put aside! Lack of bore wall thickness is the issue with going bigger pistons, for racing, 89mm is about the limit for this reason. For road use you'll probably be ok with 89mm but this would depend on the quality of the block, the boring and piston - plus there are probably greater possibilities for overheating on the road (traffic jams, inaccurate temp guages, faulty fan switches, etc!).
For greater capacity than 1800 a longer throw crank is typically used (along with con rods to suit) - so it's not a cheap solution either.