Frenchy,
Unfortunately Bob is right about the available SU 76 (1/35 scale) kits. I have 2 that I didn't build yet, but plan to shortly. I really like the vehicle and am mainly aiming to get one that looks pretty as I don't want to pour too much money into that project. I'm really surprised that the big manufacturers didn't take time to make an accurate one as this was a real Soviet workhorse during and after the war. Perhaps one of DML's mystery kits will be a brand new SU 76.
Regarding aftermarket sets for the SU 76, Eduard has on PE set and I believe MB models has or at least had a PE and Resin upgrade set. I have the upgrade set which basically includes a drop-in fighting compartment along with a few other odds and ends. I think this upgrade kit is out of production, but it wouldn't hurt to post here and other modeling sites to see if someone has this set that they never used. And, yes, Airwaves does have a PE set for this kit, but it is VERY basic; ammo racks, grab handles and perhaps something on the fenders.
I can't comment much on the tracks as I didn't take a real close look at them, but since Bob tackled this kit, he's certainly in a better position to know than I. I am grateful at least that there is an SU 76 in 1/35 scale. This gives us something to work with.
Regards,
chuck w