LPG conversion requires a lot less changes though. The gearbox and braking remain exactly the same. You need to make very few changes to the motor. Then you just add a little more equipment (fuel tank, computer and control system) on top.
So you can't remove the gearbox, or you'll have to place a new stand-alone differential, which will add significant costs.
I'm glossing over the details but you'd need about half a dozen precision machined features and like three or four parts that are specific to a particular ICE engine/transmission application. Dimensions are such that you could probably buy cheap axles for a common application and then have them machined down to make your part. Parts cost should wind up under $200ea unless you need something expensive to machine (like blind female splines).