How is it confirmed a bad engine?
From what I found when I was looking to repair my TV the TSC rep indicated the DMD board assy is not included in the light engine assy. The light engine is from the lamp point to the frenzel lens, but did not include the DMD board. This may have changed from a couple of years ago, though.
The engine, or DMD for that matter, should never go bad. At least it is designed for a long (possibly infinite) life. That being said, factors such as cold solder joints, faulty circuit card traces, marginal capacitors all affect the reliability.
Being an Optoma owner, I spend time on the Optoma thread, but occasionally browse mostly the Samsung threads as well. DMDs have failed, color wheel bearings go bad and light tunnels collapse as well. Samsung has a much better support capability than Optoma. Most if not all parts can be replaced at a shop (or at home), but the Optoma is very much a remove the entire light engine, DMD assy, pwr supply or processor board.
The RD 50 is a top notch TV, but at some point spending $750 for a replacement light engine or board and having an old TV is right on the fringe of 1/2 the cost of a new 45-50" LCD or Plasma. With new 50" DLPs starting at $1100, its hard to spend that much to fix it, in my opinion.
One other poster had problems with the light tunnel in his RD 65, and was able to get a replacement from the store he originally bought his from. Another customer's RD 50 went bad and scrapped the entire set. You may start looking for spare parts on e-bay.