In the world of home built aircaft there are many options and much to think through.
In general, there are really two key decision to make.  Certified Vs. Experimental, and water cooled vs air cooled.

Today there are many engine choice available at a verity of price points.  Proven legacy modals from Lycoming and Contentintal are avaliable on the used market.  Entrepreneurs and innovative firms from around the world have created a number of light weight high power engines suitable for this airplane.  Rotax, UL Power, Verner, D Motor, Jabiru, and Simmoni Victor, and of course, the ever present 100+ hp Corvair aero conversion.

Having worked and flown with both air cooled and water cooled engines, I can say without question I prefer air cooled.  With a water cooled engine one must figure where to put the radiators, house routing, and maintain the cooling system, thermostats, and so on.  If a water temperature gauge does not work, you have drain it, pull the sender, and trouble shoot it all.  Its a big hassle and in my opinion not worth it when there are proven and simpler air cooled engines available. 

My preference is to stick with a proven air cooled engine like C90, O-200, or a Corvair.  The Jabiru Gen 4 engines also seem to be working out very well, and deserve your consideration.   UL Power seems to be picking up steam as their fleet builds time.  If you choose a water cooled engine, my suggestion would be to seek out person who has installed the same engine in a similar aircraft, and flown behind it long enough to work out the kinks, otherwise you will be the one dealing with air flow, cooling, and electronic issues all by yourself.

If a water cooled "modern" engine is what you really want check the Aeromomentum, and Viking Aircraft.  They have done just about as much as is humanly possible to create a user friendly water cooled auto based aero engine using the best modern engineering has to offer.  They both have a growing body of installed engines accumulating time.  If you want a modern water cooled auto engine that has had much of the brain work already done, then these two vendors are worth looking at.  No matter what you choose you are responsible for the engines performance, reliability and maintenance, along with any frustration, and expenses associated it. 

When considering an auto conversion engine from a vendor its important to ask a lot of questions.  One important question is how many propriety parts are used in the conversion?  If the vendor retires, or goes out of business and you need parts where are you going to go? Are there alternative assemblies available from other vendors? 

If being your own mechanic and doing your own maintenance on a simple, time tested engine is what you want, then a Covair aero conversion, properly constructed and run according to a well known expert with considerable experience should be the only way you go about it.

If working on engines is not something you want to do, and would like to be able to have any mechanic on any airfield in America be able to safely work on your airplane, and have access to quality new parts, then the Continental C85, C90, O-200 or Lycoming IO-233 are the clear choices.  A mechanic at Hooterville county airport is not going to touch your neighbors old Evinrude 2-Stroke engine he somehow figured out how to install in your new Cub Major when it acts up leaving you stranded, but he will feel right at home working on an O-200.   

 
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