KH100 Last oversize piston??

The simplest and probably cheapest option for you, is to find another KH100 barrel that is on its standard bore size, bore it out to first oversize and fit a piston to suit. Getting your hands on a stock-bore KH100 barrel will be the hard part. If you get one from a wreckers, make sure they measure the bore size accurately for you before you buy it, in at least 3 locations from the top of the barrel to the base, around the cylinder - to get an idea of what the bore size actually is.....

You can go with re-sleeving options if you can find a sleeve for this model - maybe Nobby at Hyside Racing could help here ??

Thats bloody hard work that.
Anything from 74 onwards Kawasaki 100cc fits right through to 94 ( search UK fleabay for more current KH100)
KE, KH, KM, KV 100 barrels will all fit. Engines prefixed with G3 through G7. Difference is all in the materials and specs it is made to ie port size and timing. KV or KE 100 barrels are the go because being offroaders the were ported for a little more torque down low whereas the KH was ported for peaky traffic and city commuting. As mentioned above the KE and KV barrels were aluminium, the KH is cast iron. its another longevity thing. The KV was an AG bike and the KE was the MX'er of the day and they run at slower speeds necessitating larger fins and greater heat dissipation with low air flow.
Just for the love of god don't walk into a wrecker and ask for the barrel only. They will arse rape you. Just point to the whole pile of crap in the corner and say "owmuch?"
Match your exhaust port to the header pipe for increased efficiency. Expand the inlet behind the rotary disc - there is a lot of meat that can come away from there. Alter your disc timing. Add an early 80's KX80 electronic ignition - no charge circuit. Set squish to no less than 0.8mm - 1.0 is good for reliability. Add Yamaha kart pistons for increased redline. RG250 expansion chamber. Z200 or 250 front end. Rear sets with ZX9 or ZX6 components.................ahh fuck it you'll work it out