Xup,
Actually the engine question turns out to be a matter of personal preference and choice. I'm with Cumn thru on the Saito 100. Reliable power, great customer service and a standard three year warranty. On the UCD however due to the fuel tank location you may or may not need to purchase additionally a fuel regulator such as a Cline or Iron Bay. This problem can be eliminated by using a pumped engine. LL uses a pumped OS 91 4 stroke I believe and Autoguns uses a YS 110FZ that he raves about.

Joe is actually the most experienced modifier of these aircraft. I imagine that after he gets done installing the YS 110FZ, he will have to get a permit from NASA. Like I said it boils down to personal preference and what one can afford. If I could afford a YS that is probably what I would buy. They have a reputation for having to be "Tinkered" with, but that would not be a problem for me. OS makes a reliable product noted for running well right out of the box. My two stoke engines are all OS, I have a 46FX and two 61FX's. Both have run flawlessly after routine carb adjustments. This is my second Saito as my FA120S runs flawlessly. As you are in the UK, LL can guide you on where he got his deals. I'm sure more opinions will be posted. Good Luck and Welcome Aboard.