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.

My preference is to stick with a proven engine like the A65, A75, C85, Rotax 912 and the Jabiru Gen 4 2200 engines. UL Power seems to be picking up steam as their fleet builds time. With used Rotax 912s selling for under 10K, they are probably the best choice.

A lightly built direct drive Volkswagen in the 2332cc range turning a 62-63" prop should also proivde inexpensive, and adaquite performace.

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.  Its alos important to consider the total weight, battery, fuel pumps, ECU, and so on.  Weight adds up FAST.  The CubMajor wants to be light. 

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 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 and Rotax Engines are your best bet  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 A-75.
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