I've actually owned two, the first I bought new in '87 for 3G's. Soon there after it became a rolling test bed for after market goodies as well as homemade equipment. I'll try to remember some of the mods:
homebuilt nerfs before buying the real thing
homebuilt front stabilizer (sway bar)
home built rear lowering strut before purchasing one
homebuilt headlight out of automotive head light
mod'd the upper "J" arm into an "A" arm (should've patened it, it worked great but Yamaha eventually corrected that oversight
beeefed up the rear axle carrier mount with 1/2" bolts and a cross bar (another correction Yamaha made later)
braced the front upper frame area
recovered the seat myself
Dick Cepeck spider tracks for Hill climbs
Turf Tamers for normal trail riding
Scat Tracks for sand hills and sand drags
Hoosiers for flat track racing
(replace the butter soft stock rims if Yamaha has overlooked this prob)
Axecaliber +2 (I think) rear axle
Works Performance rear shock (POS)
painted the frame silver(the quad required so much maitenance it seems I had it stripped to the bare frame fairly regular)
aftermarket bars, Oury grips, mastercylinder guard and twist throttle
elimanated the POS parking brake and key lock
drilled the rear rotor (the second one, I warped the first)
FMF gold midrange pipes and silencers
"flexier" reed valves (dont remember the name)
K/N filters mounted on homemade ram type intake(this and the pipes really helped the low and midrange tremendously)
milled head and bored cylinders aftermarket pistons (most of the machine work was performed by Phil Darcy in Haltom cty)
aftermarket handgurads and EBC brake pads
this quad was stolen along with my truck and a TON of riding gear, the day the first Gulf war started, and never returned. this was out of a semi secure parking lot in view of the guard shack!!!!
Ins paid me 3Gs.........
The second one I found was the same year model. I saw it in a used car lot and it would not run but was other wise in good cond. (paid $700) I eventually found a loose wire was the problem after the shop tried to charge me $500 to "try" to fix it. The only mod I did to this Quad was to drill two one inch holes in the airbox to eliminate a flat spot in the midrange. I later sold it + a ton of gear and got out of the motorized off road scene. I shall return though, as soon as my three years old son is old enough to begin riding...........
BTW I got real good at sync'ing those twin carbs and also doing ring replacemants in short order.