the tabs snapping was quite a common problem with these old TM rifles. If you still have the broken tabs then you could cut metal tabs from a coke can and epoxy it on again using the coke can tabs as reinforcement.
The easier option would be a replacement metal upper and lower receiver however you will need a hop unit as well as the old TM M4's had a 2 piece hop unit that only fit the TM plastic receivers. Back in the early days of Airsoft Guarder, Hurricane and Systema made some very good replacement receivers.