I'm flying with a four cylinder Eggenfelner Subaru engine. For my RV-9a it is a great engine. There are no current firewall forward packages available for automobile engines in an RV-10 as far as I know. Even though I'm happy with my engine, I feel like it would be very risky to try to make your own conversion, or use a bought conversion without a good track record. There are too many unknown failure modes unless there are many engines with many hours of the same type and same system of ignition, fuel, electrics, etc.
Save yourself alot of trouble and in the end, probably alot of money.
-Andy