This is a very important question that needs to be answered to really give you a good response to your question. Additionally, it would help to know your budget because we are talking about engines that run the full range of price.

If your looking for a very dependable 60-size engine for basic sport or trainer flying, you can't beat the OS LA 65. I'm not familiar with the Magnum XL .61 (I have one but haven't found a plane for it) but think if it is dependable it would be a good economical engine. You opted out for probably the most dependable engine in this size range -- the OS FX .61.

The only people I have found that call the MDS 68 engine a good power house is on this forum. My own experience is 100% negative and others in my club are running the same opinion. Bar none, the MDS 68 I owned was the most difficult engine to run and for me it had less power than one of my OS FX .46s in a frame that was about 20% heavier. I had the same problem with an MDS 18. I've learned my lesson and avoid this brand like a plague.