I did a little research. If you're really on a tight budget, the Magnum 1.08 looks pretty good. It's rated at 24 oz with muffler. Street price is under $150. The Bisson muffler is $46 (you can use the stock muffler, but it will require a large ugly hole in the cowling). I think you would be quite happy with this combo. I can't vouch for the reliability or durability of Magnums though.

All the 90's I looked at were quite heavy except the MVVS. Unfortunately, it will cost about $230. You may or may not need a different muffler for this engine. The ST90 was interesting because of the swivel muffler. This would work well in an Ultimate cowling. Unfortunately, the power/weight on this engine doesn't look too good.

I was very close to trying the YS91. I changed my mind at the last minute for two reasons: 1) I got an excellent deal on the Saito, 2) there is a very large Saito experience base in my club. It turns out that the Saito 120 is a very sweet running, user friendly engine. Highly recommended.