Van's has done the engineering, design and flight testing of their aircraft using the recommended engines specified for each model. Going "outside " the box on your powerplant opens up a whole new can of worms and with that comes your own engineering, design and flight testing. Adapting a modified auto engine to your bird is a challenging task, that may seem economical on paper, but in the real world can prove to be quite a monumental undertaking. Engine mount, cowlings, fuel system, cooling & propeller, just to mention a few will need to be addressed, adding countless hours of build time. If you are considering this based solely on cost, then don't do it, stick with a Lycoming and shop around for a good deal. If however you are willing to take on these challenges, then go for it. As mentioned, your resale value will go down the toilet. The list of "successful" auto conversions Subaru, Mazda, Chevy etc: is fairly small, and many of these aircraft have since been converted back to conventional aircraft power.