Tirewise, I'll want as much meat on the road as I can get for my 90 builds. I just measured the forks on the '69 CT90 and '69 SL90. There's 4.0" [140mm] clearance between main body of cast aluminum lower fork tubes, 3.75" clearance between fender screw mounting boss without fender on lower tube, 3.5" inner clearance between stamped fender brackets on fender: 3.5" clearance would be maximum with stock fender in stock position.
Of course many dirt bike now have fenders mounted right up under triple tree, bolted to the bottom of lower triple tree bracket. Doing that on a CT90 would mean that any tire less than 3.75" total outside width will fit.
I'm looking at the IRC GP-1 tires as a strong possibility, those being a fairly aggressive trials tread with 80 mph continuous speed rating, so a good stout carcass. According to what I find on the web their 17" is 3.60" wide. Cool baby cool! Possibility that to file, trim, grind on a small area of inside of fender mount arms, 1/8" per side, might give enough front clearance with stock fender. Also possible to mount tires to bike and use an angle grinder, spinning wheel, to pare down just a bit on tire's side lugs for a tiny bit more clearance. Plenty of clearance at swingarm for the IRC GP-1.
CT90 fork tubes from axle center to top cap are roughly 26.25" length, SL90 are about 28.75", and lower aluminum section of SL90 tube is about 2" longer than CT90, same diameters all, same triple tree numbers for stem diameter, length & hole spacing. CT90 stock tire is 17" front, 17" rear; SL90 is 19" front, 17" rear; but I also have a set of Yammy DID alloy wheels [need Yammy axles] at 21" front & 18" rear that may go on the SL90. However I have a very tweaked Yammy AT2 Enduro, so maybe stick the alloys on that.
These IRC GP-1 are only available in a 'Rear' configuration. Is there any big deal running one as a front? Seems to me a lot of that 'Front' & 'Rear' designation isn't real super important with a tiddler??