What cfaw10 says is true to a point.

With the cheap Chinese engine you will always have a poor quality engine.

I like the Echo engines. Great quality, plenty of power and reasonably light.

The older 23.6 engines are real good runners and there are articals here on RCU on how and what to modify.

They are still easy to find in the junk box at most small engine repair shops, usually for free and never more than $20.00.
A lot of the time there is something other than a scored cylinder that put them there.

Bad Carb, leaky gas tank bad ignition ect....

They also run well on Gas-Glow and don't even need an ignition system.
$35.00 will buy you a prop hub and ring set up from Top Cat on E_ay.

So $50.00 will get you going with a good base engine with a new ring ready to run or, $100.00 with an electronic ignition.

This stuff is not rocket science. It's pretty easy for being hard. And it's a LOT of fun.

Starting off with a good engine is important. Other wise you are wasting your money.

It costs just as much to do a 50cc engine as a 20cc engine so don't go small.

Hope this helps......