Circa 1970 when I was a grad student at the U. of Wisconsin Engine Research Center we prepared two cars of the "Clean Air Car Race" sponsored by MIT.

One was a Opel GT that we converted to propane using a carburetor and catalytic converter common to warehouse forklifts. We also designed and built a mechanical EGR system to control NOx because we had to meet the then proposed 1975 exhaust emission standards.

The conversion wasn't too hard. I'm not familiar with "propane injection" or know if such a system exists today, but if propane carburetors are still around I'm sure one could be adapted to the manifold.

However from a practical standpoint you can probably buy a decent relatively low mileage 124 or 201 on bringatrailer.com for less than any propane conversion will cost.

Duke