ATC200X carburetor, FMF pipe, HotCams camshaft, White Bros CDI box, open up the air box inlet, light polishing of the exhaust ports, gear it 13/48. {approx. $500}
thats about all you can do without spending a lot more, and still have 100cc displacement
I would leave the XL100 alone and invest the money in something a little more recent.
Air cooling has its advantages, I never get ready to ride, find a puddle of air under the bike and spend the day fixing the leak!!
Robert