Not aware of an '89 DTRE engine.
In the UK the DTRE was from 5-07 where the bike was discontinued, but in France there was some Electric start DTR engines. But I don't know too much about those.
The best thing to do in theae situations when you're lost, is get the bike back to standard. This means fixing the sidestand switch and 'oil sensor switch' (I presume you mean the neutral switch on the bottom of the engine).
These engines are incredibly simple, so it shouldn't take too much head scratching to figure out.
