Well, don't give up just yet, you can make something like this, it isn't difficult to do:
YouTube - BICICLETA CON MOTOR 95 KM/H 60 MP/H PUSHER BIKE
(Look at his channel, he has many videos showing that thing)
Or... You could go this way haha:
YouTube - how to make a homemade motorized bicycle
Well, if done right it's a nice design already made many times (search for friction drive). That way you don't need a clutch (you just lift the engine). Off course that video is kinda a joke, but there are many nice ones out there.
I don't quite get the walk-machine thing. If it already has a engine, what would you do then? I'm lost.
In my oppinion, don't give up just yet, i think it's even simpler to do a rack-mount.