The Tower Kaos is basically a .40 sized Kaos Jr. or Jr. Kaos (whichever it was called back in the early 70's ) and a K&B .40 was typical for the day. A friend had an HP .40 w/Tarno carb on his...made a very nice combination.
Both of those are very light engines...
I have a Webra .50 in my Tower Kaos, and it is way more than sufficient.

Any modern .46 would be plenty, and with the O.S..55 it would be quite fast...perhaps a bit more than you're ready for...but...there is a throttle on the engine...

Edit: Mine wound up quite nose heavy with the Webra .50 and TT spinner. A check of the weights of your potential engines is recommended. I had to move my battery pack back behind the wing...had to cut a hatch in the bottom of the fuselage just behind the wing.