The most user friendly 40 sized engines would have been the plain bearing OS LA 40 (or 46) or perhaps the Evolution engines.

I fly the Sig Kadet LT-40 which my son and I built from the kit. It is larger than the prototypical 40 sized trainers (900 sq in of wing area) so I went with a ball bearing Thunder Tiger 46 Pro. It is a durable, well running engine with ample power but a bit more finicky to properly set up a reliable idle.