Currently there are some weapon upgrades that seem to be designed for making thrusters out of weapons. The upgrades that increases weapon recoil, and also especially the one that reduces weapon damage by 100%, those upgrades are very effective at turning weapons into engines, but are not very useful otherwise. Since it could be done with other weapons, I think flamethrowers should be given recoil as well as it would be a much better option for a makeshift engine. The acceleration from the flamethrower would be smooth, as opposed to weapons with low fire rate, it would not flood the screen with bullets, and it would be better in aesthetics as the visual effects would look just like a jet engine. Additionally, real flamethrowers do seem to have recoil so I believe that it would make sense in the game as well.
