Tower .75
Best engine in the hobby for under $100. 2yr warranty--unless your a Tower Club member, then it's a 4yr warranty. But it's out of stock and it's anybody's guess when it will actually be available again. They've been out of parts for over 6 months.
http://www3.towerhobbies.com/cgi-bin...?&I=LXUR78&P=0

The ST .75 is another option, but I'm not sure it meets your weight requirements. The muffler is heavy. If you look at the raw weight of the engine, it's just 1.5oz more than the TH .75. You can get a lighter muffler for it.

There's the OS .75. It's TWICE the cost of the other 2 engines. It's no lighter. Not the one I'd choose.