I really like my YS and Saito 4-strokes! I love the power, the fuel economy, the easy cleanup (most of the exhaust oil goo burns off in the muffler) and, of course, the SOUND!

OTOH I'd have a hard time recommending one for a first-time flyer. The engine on your trainer is very likely going to take a dirtnap at some point - all those extra moving bits are more prone to damage in that case.

All of the .46-sized 2C engines mentioned above will work well. The OS will be the easiest to get running; the Irvine and TT motors take a little more breakin time before they settle down.

FYI - I do have a YS .45 on my Superstar .40 (its the de-facto club trainer). This engine flat-out kicks the butt of the above-mentioned engines in the power and smoothness department, as it has a pump built-in. Once again I can't recommend it for a beginner, as the engine costs more than two ARF trainers!
(The only reason I have it on this plane is because the OS .46FX that was on there before wore out, and the YS was the only other 2C glow motor I had on-hand.)