MDS .58 or MDS .68 if you can spare the weight. Both less than 100 bucks new and much cheaper online. All of mine run great and never gave me any problmes.

If you insist on an OS, I'd go with the LA .65.

If you can, try to find a used ST .61. I also like these engines. A new ring can cost 3-5 bucks and you can get a good deal on one. Make sure you buy one with good bearings, I don't care for replacing them. They will last almost forever.