There's quite a few of those LT500R rear cal's on fleaBay for under $40.
I've owned many a rear caliper from the LT250R/500R. With my LT500R the braking bias is like 70% front and 30% rear. Those front disc brakes are the ticket. It would be worse with the Oddy give the weight.
The TRX450R rear caliper is similar too. Actually any of the single-disc rear calipers would potentially work as long as the p-brake is on the same side. It all comes down to the brake stay / mount. If you don't need the p-brake (use a line-lock), maybe there's a 2-piston front caliper that could work. All comes down to the bracket.
Same with the front discs. If you get the 86-87 TRX250R front hubs with discs, you can find a small enough caliper to cram in there (e.g. Polaris Predator calipers fit for me). Then it comes down to the bracket. I started making some of those brackets but that's been on the backburner for what seems forever b/c I needed to rebuild the master, then wanted to replace the master with the twin masters, etc. Still on my to-list now that I've got a cnc plasma, but been too busy with artsy stuff and fab/farm projects (and kids, and work, and...I need drink lol).
