On looking at the 1/2 A site tons of posts on coxes, norvels, a few on APs and CS > there is lot of stuff avaiable for cox
reeds, diesel heads, cranks from Davis. Sadly unless you can adapt the front intake to accept a small RC carb (where do yoo find one?) Davis has a very workable solution by connecting a servo to move a rod to vary the compression screw
still a exhaust oil issue blowing over the plane in glow or diesel

The norvels are no more and the APs and CS leave a lot to be desired for quality control

Of course Burford makes a very high quality engine but the price is out of reach of many ( yes the engine is worth it)

There seems to be a few of the guys using the TT07 inspite of its low power it runs fine and holds up
The surprise is PAW references they have the engines (this may get a few more into diesel) but a little pricey

With the tons of nice ARFS out there some are looking to convert from electric to glow or diesel
and of course the kit and scratch build guys looking too martin

Once you hit the 0.10 size you are ok either as glow and of course the OSs and TTS have diesel heads available from davis if the airframe can accept the size and wt of the larger engine