Wow...sounds like a very cool project - something I would love to hammer on in the twisties. There's nothing quite as much fun as a 2-stroke when it "hits the pipe" and switches on.
As far as economy goes....hmmmm.
The CR125R 6-speed transmission has very close ratios, so not good at all for economy.
Also, remember that this engine produces power like crazy (~40 HP) in a very limited rev range. In the power band, it probably has a better BSFC than expected, but outside of the powerband, the BSFC is absolutely horrific. 40 HP is too much for moderate acceleration on a light bike and you will have to use most of the 40 HP in the "high BSFC zone" to extract the most energy from your fuel. This will mean frequent pulsing (for very brief periods because of all that power), combined with even more EOC. The good news is that bump starting will be easy because of the easy turnover of the 2-stroke.
EDIT: Duh...I just reread your last paragraph. You don't care much about mpg, so disregard all I have just feverishly typed!
