Well, I've got a rebuilt (by myself and The ManxManiac) 1914 in my current 'project' manx. So, right now, I'm going aircooled, normally aspirated, little more HP than a stock 1600. However, I sure do like Robert Wilkes, Hondratti El Lobo, long travel buggy. A Honda 3.0L V6 (200HP & 200FtLbs) with a 091 transaxle, solid and dependable to my way of thinking.
Northwest Sand Gear sells the Honda V6 with a computer for about $3,500-$4,000, the 091 should be about $2,000 with the Kennedy adapter plate and clutch. Don't know enough about the Subaru engine, but I think it is a proven unit as well (it's in Bruce's Dual Sport Manxter). I just haven't seen anyone selling it at about the same price as the Honda (I think I heard Bruce say his Subaru was about $15,000). I do see them on Ebay for about $1,000 without a computer, but, I'd like to get the turnkey engine solution. Looks like it requires more mods than the Honda, but, I like the flat Subaru engine form, tucking under the rear, better than the vertical Honda V6 that requires body mods. Go with what you want, is what I say. Good Luck.