There are aftermarket kits out there and it is feasable as Propane it
a very good motor fuel and the cleanest burning one  there is too
currently. Dual fuel is nice but bear a few thing in mind. Propane has
a lower heat content per gallon compared to gas but more than pure
alchol and it is a lot lighter in weight than both of them. Since it
has less energy per gallon you will use more fuel than with gas,
(about 30 to 40% more) on a dual fuel engine.  One real nice thing
about propane is that if you want to go propane only (a big step) you
can rebuilt engine to very high compression for more power and
efficency (and MPG) and even super charge it to without worry about
knock because Propane has a octane of about 110.  It is possible to
get some very impressive power from propane only engines and there
were a lot of farm tractors built in the late 50’s through 60’s that
were built to run only on propane when it was really cheap back then.