My recollection was that the XL75 was a larger bike than the XR75. More akin to the SL100/XL100 size, with 19/17" wheels, rather than 16/14".
Could be wrong, although I do recall seeing adults riding them without looking too silly.
Of the bikes mentioned so far, what's a rough order of competitiveness for mechanically stock examples?
My guess of the bikes I think I know anything about:
YZ100/RM100 (if you allowed pre-78)
Daylight.
TM100.
Daylight.
MX100.
TS90MX.
SL125.
DT100.
TS100.
LT2MX.
SL100.
TS90.
LT2/1.
Keep the jungle-juice out of it. The bikes that currently exist that run on it were built to fit into different categories - and will be free to keep running in those categories.
Maybe a rule that out-laws anything that was originally fitted with a 21" front wheel would be a good idea??!?
Actually, maybe that's all that's needed - limits on engine capacity and wheel size?
?
The SL125 has a 21 though, doesn't it?