I take my wife over to the local thrift stores when I go browsing for frames. The bikes are always less then $10.00 and we seem to get great deals for the kids. So, as long as I let her pick up some knick-knaks while we are out, she could care less on what style of bike I bring home. All my kids have really super rides, and non over $15.00.
I also chech out the garbage sales on the weekends, then go by the locations on Mondays, or at least call on Monday, before they call the local thrift store to come and haul their junk away. I bought a real nice mountain bike / cruiser for my 9 year old daughter last winter, only paid $5.00 for a $150.00 full suspension, 21 speed kids intermediate rider.
Now she has seen some of the motorized rides and wants the conversion to her bike.
So, you have several options. First plan your coarse, meaning draw it out and write it down on paper. Nothing like NOT having a definate plan/ blue-print to your intentions. Or, you can get creative and go purchase materials to build your special dream machine. Only makes sense to have a welder handy and table to build a jig/ fixture so you get it all correct.
This actually saves the time of finding a sturdy chassis from all the running around to the local thrift stores.

I prefer flea market ventures, it sometimes pays off, and other times the wife wins 100%.
Good luck on the adventure, I could go on forever, but I am sure you have a good idea brewing for now. Have a great day, Maximo