That is a good choice. I would put the sensor pick up on the prop hub though. I would also look at loosing the rear part of the crank and little bell housing. I Would see if stand offs or a little plate would work. Basically save all the weight you can.

Here is a nice echo build log:

http://tech.flygsw.org/echo_conversion.htm

Oh, PS, the john deere looks like a homelite, thanks for the offer but, give it to someone else if they want it.