You can use any engine with horsepower between 160hp and 235hp. You can use a conventional aircraft engine or an "automobile conversion" engine, as long as you stay within the horsepower bracket mentioned above.
For flying on wheels only, a 160hp engine can do the job if you are concerned with fuel burn. A 180hp however would give better performances.
For float flying, we suggest a 200hp or more. We have seen a big difference between the TUNDRA number one flying with a 180hp and a fixed pitch propeller, and the TUNDRA number two, flying with a 200hp and a constant speed propeller.