As with any motor of this age - compression should be checked.
The main bearings really should be greased. This includes splitting the case.
A good precautionary job would be to grease the bearings and install new pistons and be done with it.
Also - the carb boots are junk. Look them over. They may be cracked badly. Some don't tho. I have been swapping used boots for several yrs now on my 02 (800) but from what I gather - the "fix" is to buy a new set of boysen reed/boots assembly. (Supposedly the most reliable fix - not neccessarilly the biggest HP. ???)
I cut down some rods for DJM to stuff a Kitty skid up inside one of his Doos. For some reason I still have the drops here in the office, and they look to be about 3/16" wide each - not counting the kerf from the saw or cut-off blade that I used. I am sure it will fit if you want it to, but of course the mounting points are likely different, so you would hafta doo some measuring and drill new holes. You would have to doo a search for DJM as he seems to spend most of his time over on Doo Talk these days.
I have a riding chumm that has an '02 700 Renegade with the 136. He rides with us longtracks (151 and 156) and many days in N Ontario he does at least as good as us! Especially on the heavy (wet) snow days. It is actually quite embarassing. However - I am sure that if we rode in some bottomless stuff one day that he would be hurtin a bit.
You could simply slide buy longer rails off of a wrecked or junked 136" ZX sled and bolt on your sled and be to 136". Check with Al's Snowmobile Parts in NY I think. ??? They will be able to set you up with anything you need!
.