If your looking at using the CH igntion...
I'll second the little 33/34 cc poulan...

I have a Poulan 2150 34cc I bought for 40 bux...and it runs like a little raped ape...

You gotta stay with the PORT inducted conversions for the extra punch....chainsaws are the way to go....think about thier normal use of operation....string trimmers (governed/regulated/reed valve induction for longevity)......chainsaws....on the other hand..all out wide open baby!!!

They just have a whole different "feel" to them....

Not to bash the newer twin ring ryobis though...I have two....can't wait to get them flying...

When i ran the two trimmers right after each other, you could tell just by the sound .....twin ring is much "pop"ier sounding....cooolll!!

Best of luck, keep us posted!!
CrazyHerb