There aren't any huge reasons not to look at that engine, but when mated to a proven airframe like the RV, there are a few considerations as mentioned previously. Given the market right now, in order to have an nearly "one of a kind or one very few" RV (out of thousands of thousands flying) the price should reflect that....meaning that an RV6A with that engine installed on it should be REALLY cheap as compared to it's Lyco powered brethren.
Also check with the insurance company to see how that treat it. Might not be an issue at all, but it is "non standard" to an extent.
As others have said it could maybe be a good deal, but it's not common in RV airframes. Therefore it's quite an unknown to most of use that have been around the RV's for a few years.
I guess before I opined either way to strongly I'd have to know a lot more about the airplane, the builder, the engine, the prop, the price, etc... Any of those factors could convince me positively or negatively about it. At the moment I have to be somewhat ambivolent because I don't have the data to form a good opinion.
My 2 cents as usual!
Cheers,
Stein