If the advertised weight is accurate then you'll be fine with a good 50-55cc. The 85 might be beneficial for serious 3D but otherwise overkill.

The DA-50 is an excellent engine. If you don't want to spend that much though, look at some of the low cost, high performance 50cc class of engines coming out of China. Some of them run very well. Tower Hobbies is selling the DLE's now and Hobby City has some high performance 50's that are dirt cheap and perform great considering the cost. You'll of course give up some of the support and other perks of going with the more expensive engine but you might be fine with that as long as you know it going in. You just have to decide what your main requirements are and buy accordingly.