I really dont think it is possible to achieve proper vehicle sound without FMOD right now. The best you can get is what we have in the vehicle game project and it is not great as you may already know.
I suggest downloading the sample FMOD UE4 project from their site and take a look at how it works in FMOD Studio as well to see if it will work for your custom vehicle. Normally you need several engine revving sounds for each gear, and Fmod handles the gear shift depending on load value. If it was a normal car i would definitely say go for it, but you’ll need to see for yourself in case as i dont know what you can replace engine rotation speed with.