Peter, I'll repost my response from the Scale Aircraft thread just for the discussion:

"The Ryobi 31cc is a popular choice and should power your B-25 well in a scale manner. I recommend getting CH Ignitions for them so that you can have to two engine counter-rotate. The Ryobi 31cc is reed valve intake and will happily run "backwards" if you set the ignition timing correctly. The rear intake rear exhaust also make it very easy to conceal inside smaller cowls"

The reliability of these conversion engines lies in the ignition system and the carb. The basic engine blcok itself isn't likely to develop problems in RC use, which is super light duty compared to the weedie and chainsaw environment, for which they were originally designed. Now the carb: sometimes these pumper carbs will have problems with getting a steady atmospheric pressure to the diaphrams to pump and meter fuel properly. With the Ryobi, the carb sits behind the crankcase and so will typically be shielded from much of the turbulence up front in the prop wash. I would consider that a plus for reliable operation.

As for the ignition, the stock magneto ignition is just about bullet-proof. Provided you set the air gap correctly, not much could go wrong with them for hundreds of hours of operation. In case you upgrade to CH Ignition, well..., then you have identical system to some of the best gas engine available for RC use (BME, FPE, Brison, Taurus, etc.)

Power from the newer twin-ring Ryobi 31ccc engines are substantially greater than a G23. I have a Ryobi 31cc, Homelite 30cc, and a G23, all twin-ring, and all on CH Synchrospark ignition. The Ryobi 31cc has the most power. The reed valve allows the Ryobi to swing a larger prop than the piston-ported ones. This might be important to you on a scale aircraft. A B-25 should not be expected to perform aerobatics, so the too often overblown overpowering mentality should be kept in check. I'm sure you may already have read this article, but it does give you great starting point:

http://flyinglindy.homestead.com/b25construction.html

In short, I would not be afraid to use any of these popular conversion engines on a giant scale B-25, as long as the conversion is done well. I do recommend that you purchase brand new engines, instead of trying to save a few buck with refurbished ones though.