One of these 66cc engines, completely standard, with no modifications not even port matching, makes two kilowatts on the Dyno. Given that this is ten to twenty times the output of a comfortable human being, asking "what do you do when you can't pedal fast enough to keep up" sounds incredibly daft.
You might as well ask how you row a cruise ship or a speedboat. The answer is you just don't because its totally redundant.
And nah a bike doesn't need anything there to stop it from dropping a chain. There's millions of bikes out there with 1x drivetrains, that was the cheapest and most common derailleur setup for probably 30 years continuously and none of them had any kind of chainguide. Guards, yes, but that was only about not getting your trouser leg chewed up. Real cotter pin days.