Young'n dumb, this is your lucky day!! That year is the best blank canvas you could have!
For heads, you can use the early single-piece heads which have tremendous porting potential, and can be fitted with larger valves. There is a very wide selection of aftermarket cams from mild to wild for that head.
You can also go with the later 2-piece heads that came with larger better ports and larger valves right from the factory, and still have potential. There is a slightly less-wide selection of cams for those heads, primarily because those heads aren't legal in most of the vintage race classes, so they kinda get ignored for track use.
If you go with the later heads, all you have to do is use the cam chain and tensioners from the laters years because the cam sits a bit closer to the deck in the later heads, which means those later pieces are a bit shorter.
As for bore and stroke, Powroll offers several big-bore choices, including higher compression versions. They also offer stroke services that, combined, will give you nearly 200cc!!
It is also possible to mix 'n match other odd pistons (SOHC 750 pistons) with custom bores and get 140 ~ 150cc, but I don't remember the specific details. Search those other sites I mentioned.
The 2 biggest problems with the 100 is that it has a 1mm smaller-diameter wrist pin than the 125, and I think a couple of those big-bore kits require the larger diameter pin. You may have to go with a 125 crank 'n rod to bring that off on your 100. The 2nd problem is that most of those big-bore kits require the 100 cases to be bored slightly to accomodate the larger cylinder sleeve. Not impossible, just another thing to chase down.
All the stuff is readily available online and, really, your wallet is the limit...
Good luck with it, whichever way you go, and be sure to post back with your plan!
Kirk
