Excuses/excuses.
~2 years back I built a XR75 for my then 3 year old grandson. He is five now so he will probably get to ride it himself for the first time this summer.
At the same time I picked up an XR100 front end, triple clamps and tubes because I heard they were the hot set up for an XR75. I figured somewhere around 8-10 years old my grandson may want an upgrade. I haven't installed them yet naturally. I did check and it looks like the they will bolt right up.
Now for the excuses part, I don't remember the year probably around 1980ish. They are 30 mm diameter vs. the stock XR75 27 mm. The length is ~4" longer which means everything else being equal they will raise the front that much. I don't know how large of front wheel you could use, I'd imagine up to 21" but that would have the effect of raising ground clearance even more. I don't know what modifications have to be done to the rear in order to keep frame geometry to the point where you won't have serious problems. Cheating a bit I imagine you could just set the tubes high in the triple clamps and install longer shocks in the rear.
Rick