DLE 55cc from an authorized dealer.

1. Great Power
2. Can be had for around 355.00 this time of year.
http://www.troybuiltmodels.com/items/DL55ENGINE.html 409.00 -55.00 savings
http://www.valleyviewrc.com/DLE-55cc%20gas%20engine.htm
3. They have a proven track record for several years.

I would buy from either Jody at Valley View or Troy built. Links above.
DA has a new 60cc engine coming out but it is more $$ and there are other 50s out there in the DLE price range or less but most don't have a real established track record. Both dealers above service what they sell.