Re: Tandem owner - new here
howdy mac, welcome to the forum.
um, i'm pretty sure none of those small bike frames will fit an engine. as for the tandem, you could cut that middle tube in the back like scotto did, and stick a big honda gx160 in there with a qmatic transmission. that would require welding, and using a cutting wheel on a grinder. you'd have to weld a tube into the corner that looks like this: \ . then your 4 stroke mounting plate could go on in there. for more ideas look around and use the google search feature. i've never done a tandem so i'm mimicking scotto's design.