You can thank the US EPA for the extremely limited amount of alternative fuel conversion kits being on the market today in North America. Before the EPA became so heavily involved with requiring conversions to comply with ever tighter emission requirements, propane conversions were a relatively straight forward affair. While never really a bolt-on upgrade suited to the backyard mechanic, a propane conversion was simple to do. Emissions weren't a problem because a propane-fuelled car would have better emissions than the same gasoline-fuelled, carbureted car. As a result of these EPA regulations, a huge number of companies went out of business or had to radically change their business plan. The installation of a non-certified system can be considered as "tampering" by the US EPA and CARB, which implies that universal systems are only recommended for off-road use.
The only kits available today to the backyard do-it-yourself mechanic are for off-road applications. A few companies offer DIY kits but these are often tailored to a very limited number of pre-1986 off-road vehicles. Technocarb currently offers both bi-fuel conversions for fuel injected gasoline vehicles and dual-fuel conversions for diesel vehicles. For current model year vehicles, kits are available for the fleet owner and these kits are generally tailored to a very limited number of fleet-suitable vehicles such as the Ford Crown Victoria and Lincoln Town Car.