It's not impossible but I think you would spend way more to convert a guzzi chassis to running a quad motor and then converting to chain drive and flipping the drive side without serious lathe, welding skills. Sometimes the most expensive projects are the ones sitting outside you got for "free"
I think your original plan is the best. Buy a used honda scooter fir cheap and then adapt one of your side car rigs to fit that. Should be fairly straightforward assuming you know how they all hook up, I don't have a clue. Theres a guy who built a fz1 with a side car as he was disabled and it was just brilliant. Very well done, he was in a wheelchair a well. Might be an inspiring read. It's in this sub forum but I'm on my phone and can't search easily. Shouldn't be too hard to find