My vote was for reuse old engine while still having trainer. The reason why is because I got a 46AX which was transferred over to my Four Star(which I had an unfortunate over-g incident and the wind snapped off) and then I moved it over to my third plane, the Pacific Knockabout. Now that I have got bored of that plane the engine is now on my Twist .40. As you can see the engine works fine for all my applications, so call me a cheap ******* if you want but the "if it ain't broke don't fix it(or replace it in this matter) stands for me. So I guess it all depends on how deep your wallet goes, as for me my planes get hand-me-down engines. But I'll definitely be getting a Saito 4 stroke for my Edge 540 later down the road. So all-in-all a good lasting engine can get you a long ways versus a throw away engine that will wear quicker.