New EX21 Electric Start engine is $550. If I do the conversion work and make a muffler there would be a discount. Engine would be $495, conversion work $125, Muffler $165, if you wanted me to do the wiring harness w/ battery add $180, my fiberglass muffler cover is $135 and you would have to install that yourself since I know you wouldn't want to ship your hood to and from.
If you want to do all the conversion work yourself, and don't care about electric start, I can sell you a used, in good condition, pull start only EX21 for $300.