just saw you're question about a larger cog on the rear wheel... yes, it'll make pedaling easier and it'll also make your coaster brake work better, 'cause it gives you more leverage. i put a 20t on the rear of my 24" and almost flipped over the bars the first time i stopped. it locked right up. took me a minute to get used to it, and now it's great.
as far as pedal extenders, as far as i know, all one piece cranks are 1/2" thread for the pedals.
they only extend your pedals out further, they won't help if your cranks are hitting the engine. not sure which problem you're having. are the cranks under the motor, but with a pull start the pedal doesn't stick out far enough for your foot? if that's the case, pedal extenders would be the way to go. but if your cranks are hitting, then you need wider ones.
in any case, here's the cranks i was talking about. if you think one might work for you, they're yours, free of charge...