I'll through this one into the mix! (Hope I don't get thrown off the forum!)
Air cooled is great, but I'm looking at putting a 1.9TDi (VW) into my bay. Costed it up and looking at about a grand to convert, however, reading a guys conversion on SSVC it all fits with standard VW parts (keeps me happy) is economical @ 50mpg, and converted to run on recycled veggie oil would payback quite quickly. Looking at fag packet maths, circa 5000 miles a year, £1.05 / litre, 25mpg = £935 in petrol per year. Diesel / Veggie conversion = 5000 miles a year, £0.4 / litre, 50mpg = £178. Therefore £1000 conversion - £935 petrol saving + £178 fuel cost = first year +£243 then every year after = £935 - £178 = -£757. so at end of second year you're £500 better off and year 3 onwards it's nearly £750 better off! I know it's fag packet maths, but this is a "green" option using recycled oil, plus fuel is never going to drop in price so as the prices go up, the difference in recycled oil vs petrol goes up, so........
Like I said, hope I don't get thrown off the forum ......