There are a number of sites on the Internet dealing with these little engines used in ultra-lite aircraft. I read on one, that there is a Mazda starter that with a little work will fit this engine.
I used the 24 volt unit and just jump it off a truck if it's home or two batteries in series. Doesn't take much to spin these engines. Or buy a 24 volt single battery like the ones used on the 5 kw gen sets.
You may be able to have some auto electric supply/re-builder re wire the 24 volt unit to work on 12 volts. There is a seller on E-bay that sells the starters for ablut $75-80, but they are listed under the 5 and 10 kw gen sets. Same starter, just need to "re-clock" the nose.