I don't know how much you aircraft weighs, but I am running a 62CC Taurus in a 1/3 scale Laser 200 (21 lbs) and it accelerates upward at 1/2 throttle. There is absolutely nothing wrong with a 3W 75. It is a very well made and powerful engine. 15 gallons (if you have not mistreated it) is barely running the thing in. Send it in for service or do it yourself. Maybe it just needs new reeds or a new plug (that little white thing in the top). Don't throw out a perfectly fine piece of German machinery because it needs a little servicing. A 1/3 scale model does not need a $1200 100cc engine.