That is a great cost analysis However, I found some information to answer this,
I don't know what the life span of either of the engines is.
I was looking at the Robins/Subaru engines here
http://www.staton-inc.com/Details.asp?ProductID=3136 and going to the other page that this vendor has,
http://www.staton-inc.com/Chain-Driv...sp?ContentID=5 I found this information.
The Robin - Subaru, Honda, Mitsubishi & Tanaka engines are rated for 600 + hours of use. We have had many of these engine run 2 & 3 times the rating. Engine life in miles. 600 hours @ 30 mph = 18,000 miles. Your engine could get as many as 54,000 miles or more
So, yes, it could give as many as 54,000 miles; but, it is rated for about 18,000 miles at steady full speed running. At a replacement cost of $200 this gives us a cost of $.01/mile.
So, add another penny a mile for the engine itself and treat it, in the calculations, as a consumable.