I salute you for taking on a challenge like this but honestly this one might be destined for the scrap heap/parts bin. By the time you bolt the few remaining pieces on a new frame and purchase/install new parts (the tupperware isn't cheap), you probably could have bought a really nice used (but complete) unit.

Not trying to deter you and maybe RTs are scarce in Poland so this is the only way to go but that really does look like a lot of expensive work.