These sets came out in the late 1980s so original parts are unlikely to be around. Bachmann Branchlines (Bachmann's UK brand) do still make Royal Scot class locomotives - but I have no idea whether parts for modern models would work with yours. Note that Bachmann Branchlines are British OO i.e. 1/76 scale on 16.5mm (HO) track. Although marked as HO I suspect that yours might also be British OO. They run on the same track.
Hamish
Hamish