I remember a guy in my class at high school built something like that, except that he put the engine in the triangle and replaced the rear wheel with tracks from an old snowblower. It worked and he still lives today. Wish I had some pictures of that crazy creature.
https://www.ebay.co.uk/itm/Petrol-mountain-bike-project-50cc-engine-with-gearbox-Sturmey-Archer-hub/302779233736