"it's an old 40 size trainer". Guess that means you don't know what airplane it is. If you post several photos that show the entire plane, we will identify it for you. Also measure and post the wingspan size. Now, as to the Dremel it or drill it question; when doing the repair to the wooden engine mount we want you to size the holes for the dowel inserts with an accurate, correct size twist drill bit and that will have to be driven by a hand held pistol grip style electric drill motor (battery or 110 volt it doesn't matter). A Dremel tool won't be capable of this because it won't have an adjustable type chuck to take the size bit you will be using. That is what G.B. is trying to tell you is a Dremel is the wrong tool for the dowel insert repair to the mount beams. Next, after you have repaired and fuel proofed the engine mount beams, then when you locate the engine position to drill the new engine mount holes it is very important to drill the 4 holes very accurately on center with the engine holes. Yes a small diameter, 1/16' or less, "pilot hole" should be drilled first so that the larger and final size drill will follow the pilot hole and not drift to an off center position. The best way to get the all important pilot hole drilled on the exact center of the engine mount holes is to use a Great Planes "dead center hole locator". This is the best $7.99 model tool you will buy ever! (#GPMR8130 from Tower Hobbies or your local hobby store) With the engine clamped in place the $7.99 wonder tool will drill a pilot hole in the center of each engine mount holes that can then be opened up to the final size needed using a drill motor NO DREMEL on this job!....clamp engine, drill pilot holes, remove engine, drill final size holes as needed, fuel proof holes and install engine....got it? Other methods of finding the center for the pilot hole can be used, brad point drill bit, or special center punches made to find and mark the center of holes. Not likely you have these or the $7.99 wonder tool either. Doing the best you can by eye will never be even close to as accurate as the dead center hole locator will be but that's what you will likely do. Take your time and make them as centered as you can for this job and later "blow" $7.99 for the wonder tool (you will thank me when you see how handy it is for mounting servos, engines and mounts, and anything else you screw down on your models).