Depends on the ubiquity of the model as to whether spare parts are available. This is something Bendigo tram workshop does. It's relatively easy to fix up exteriors since many are made of wood, but controllers, bogies, motors are really expensive if you have to reverse engineer the original parts and do new metal fabrication. Old stuff tends to be made of massive quantities of steel, iron, copper etc. so it's not cheap to make new. Melbourne's W class trams were so ubiquitous it's relatively easy to find a parts donor. Other models may not be so easy.
Also you need powered overhead if you want a functional tram and the transformer gear plus power bill isn't going to be cheap. I haven't heard of any personal tram collections so I'm going to assume it's not feasible for anyone except the extremely wealthy to do.
If you do manage to fix the tram up you probably won't be doing it on your own since if anyone is interested in the tram enough to want it when it's done, they'll probably fund you or donate labour to you to get the project done.