Check out www.altfuelconv.com
I haven't ordered any kits yet from them but I talked to the owner at the GIE Expo last Oct. He said they have custom built conversion kits for almost any gas engine. They also have tank holding kits for your mower. I plan on getting my Honda walk behind conversion kit from them eventually and the price was about $300 at the time I spoke with him last year.
The conversion process is simple so I can't see why it would be so expensive. I have a Dixie Chopper propane with a Generac engine and so far(2years) it has been great.
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