A .52 4-stroke should fly similarly to a .40 ball-bearing 2-stroke while a .70 4-stroke should fly similarly to a .46 ~ .52 2-stroke. A 4-stroke engine will be quieter than a 2-stroke, provide better fuel economy, and spit a bit less oil on your airframe than a 2-stroke.

YS's smallest 4-stroke, the YS .63, is a supercharged model that puts out far more power than your Kadet would ever want. I would rule this engine out for your trainer. Saito and O.S. both make very good 4-strokes in a .56-size, but they're breath-takingly priced at $249.99 and $194.99 respectively. The Saito .56 is actually not wildly overpriced, but there are some good lower-cost alternatives.

The Thunder Tiger F54-s is available for $179.99 from http://www.thundertiger4u.com, http://www.infinitehobbies.com, or http://www.ehobbies.com. You can get free shipping from the first two resellers, however, so that gives the TT F54-s about a $25 price advantage over the Saito after shipping. This is a very high-quality engine at a pretty reasonable price.

The Magnum XL .52 RFS is a .52 4-stroke available from http://www.hobbypeople.net or from http://www.towerhobbies.com for $129.99. At this price, the Magnum .52 RFS is barely more than an O.S. Max .46 AX. While Magnum Engines haven't always enjoyed the solid reputation that they now have, this particularl 4-stroke model is rated at a very solid 4 out of 5 stars here on the RC Universe User Reviews section, with only one reviewer giving the model a particularly bad rating:

http://www.rcuniverse.com/product_gu...?engine_id=171

Any ringed engine will require a more thorough breaking in than an ABC-type 2-stroke. I believe that the Magnum .52 RFS, if properly broken in, will prove to be an excellent match for your Sig Kadet. At $129.99, it's also one of the best 4-stroke values on the market.

If you're just not sure about Magnum or Thunder Tiger, then I'd recommend you spend the extra required for piece-of-mind and buy the Saito .56.

Good luck and good shopping!