The TLR is more of a TRIALS bike than a trail bike. The rear suspension can be firmed up by adjusting the preload on the rear springs - look at the top of the spring for a sleeve that has "stair-steps" in it - you turn that sleeve to a different "step" to change preload.
You want to know mods, I believe you can use old XR200 parts of the same era. I would recommend a UNI air filter (and make the airbox opening below your seat as big as you dare), a plug check to see how your mixture is (might go with bigger jetting, especially with airbox mods). Don't mess with the exhaust - the TLR exhaust is hard to come by in working order - you mess it up and you might be hosed.
You got yourself a mighty fun playbike there.