Heck, these lads don't even laugh at ME, much, so it's likely your little XL won't draw ridicule. I've appreciated the lack of label-*****s among the membership here.
At least in the US, for sound regs, the XL's had a comically small ID intake manifold. Like 16-17mm. Maybe yours don't, but it's worth a check.
XL has the wide-ratio 4-speed, pretty awful. 5-speed should fit. Late 75/80 has the better 5-speed. 100 has creeper first, OD top.
I've fitted plenty of the jugs with KLX110 sleeves, 62mm OD, and 57-58mm KLX-type pistons. 106-115cc depending on bore and crank, before stroking. I posted a thread with pics, some where on here. One of the less-lazy-lads might point it out
Gotta switch to external head oiling with that big a case bore (or move the oil hole to 100 spot), but you can do a remote filter and cooler that way, too. Piston squirter for durability, if you add the Tak pump.
XR100 cam, as stated, has a MM more lift, and is good for moderate builds. A little port work, back-cut valves, multi-angle seat job help make up for the little valves. Slightly more duration on the TB cam. Web 69/68 is my favorite for a stouter build. Requires spring/retainer kit, to avoid mashing the exhaust guide and seal.
Late CDI ignition can be fitted, or a Lifan adjustable outer rotor for some zing.
Almost easier to fit a 100 engine and 115-120 kit and be done with it, if you aren't in to major overhauling.
100 crank will fit any where the 80 one will, with main bearing swap, and is easier to add Akunar offset pin to for stroker on the cheap. Use the 26/25/26 KLX special pin, 94mm rod, and have 51mm stroke in a stock 100-height jug. Gotta make sure the crank and cam sprockets math out, as the 100 uses a bastard 15/30 combo.