I used to have forklift with Nissan engine converted to propane.
That thing was about 30 years old, sitting for 6 months at the time, yet starting after couple of seconds every time, beside the time when I wash the engine and got distributor wet.
Good part of propane is that is not aging, what on rarely used forklift was big plus.
Negative part it was hard to start in cold weather.
I know professional systems in cold weather use gasoline to start and after the engine reaches certain temperature, it switches to propane.
Modern car conversion in Europe runs about $3000