the diesel engines used as model engines run on ether and oil and really lousy as throttlable powerplants -
I tried a conversion years back and the inability to control point of ignition made it not suitable .(compression controls ignition)
Glow engines are a step up from these and tho the nitro "softens " the critical nature of the glo engine - it is still -in my book NOT the ideal engine
the new generation of spark ignitions is really getting good and on alky and a tiny splash of nitro - are extremely smooth runners . The problem--is thay use lots of fuel -compared to gasoline .
a lightly stressed , large displacement, mildly tuned, two stroke , spark ignition is -in my book the best possible setup.
It is all a compromise -- Don't compare these to converted weedeaters --
If you have not actually ran these and flown them -don't be too quick to dismiss em .
40cc engines seem about right for the current 5 k weight limit-I did a few models with this and I thought they were terrific - others said they were grossly overpowered when they tried em - but that was -at the time -the objective. Noise and vibes wer also a problem -I think I am close to correcting these last two problems - A new ignition system is on the horizon and the carbon fibre exhausts are working extremely well.
By the way Ed Skorepa is doing the carbon fibrework (I won't touch that stuff) the ignition is a Czech design- the engine a lightly modified ZDZ rear intake /rear exhaust.
OR you can go with electric motors - The power curve of these is the direction we are trying for.