seemefly,

I have the OS .46 ax and it is a fantastic engine.
I had the same question as you, put it in the old plane or wait for the new one and I think you should stick it in your old trainer and literally fly the wings off it. The chances of damaging that engine are small and when your new plane is built you can easily move it over. The engine should be nicely run in by then
Do a few flights on a buddy box if it will make you feel better but flying is like riding a bicycle, at least for me, it comes back quickly. Just do a few gentle circuits and touch and goes until you feel comfortable again.

Ray