Helvit, it is somewhat comforting to know you are also going through this "wringing out" process.......it can be somewhat tortureous but fun, too.
I have done lots of research the last few weeks on possible engine applications for the "Victor Slocum". One of them was the use of an air-cooled engine that was normally used to power a generator. I found that an engine of this type has no internal balancing normally found on general purpose engines and depends on the generator's armature for the balancing effect so you should steer away from that. This engine was the Honda GX360.....the General Purpose GX360 would have been okay but is no longer available....there is a GX360 available at Plano Power Products in Plano, Texas but it is the generator version so stay away from it or any other engine of 12 hp or more that is specifically intended to drive a generator.
Then I looked at Briggs-Stratton's 5.5 hp tiller engine which has an auxillary PTO shaft that turns in opposite rotation to the main crankshaft(are you listening, John Hastie?). This looked like a possible forward/neutral/reverse application by using a double pulley idler which moved from side to side engaging (tightening) either of the two drive pulleys......BUT I found that the aux. PTO (which worked off the camshaft) was only rated at 1/2 hp indicating, I suppose, that it wouldn't be satisfactory for reverse.
Then I talked with Newage Transmission Co. about using their PRM-80 marine gearbox ($870) coupled to a Honda 8 hp air-cooled engine. The Honda idles at 1300 RPM....Newage (Jamie Pratt) said this was too high and would most likely invite "cone lockup" and they "would not accept application for such an arrangement for warranty".
Basically I have been scared off of using an air-cooled engine for the "V.S.".......too many comments about danger of gasoline fire, the difficulty of adapting some sort of forward/neutral/reverse arrangement, "the engine will run too hot", etc. etc. I ran down many alleys with this and always, with no exceptions, I found reasons not to do it so I have......up to now, at least......been thinking that the 1-cylinder water-cooled marine diesel or the Blaxland Pup (although it is gasoline) is probably the best way to go. But now I'm hearing all this about 1-cylinder marine diesel vibration and have been told that a 2-cylinder marine diesel is much smoother. Then I've learned that a diesel needs to work hard and at its rated output and the "Victor Slocum" won't present enough load to keep a 2-cylinder happy soooooooo what the hell is a man gonna do??!! I think, before I lay my money down for any marine engine, I'm gonna have to see and hear her run first.
To sum it up, the lack of a nice NEW low-powered (around 5- 8 hp) smooth running inboard gasoline marine engine with flame arrestor, gearbox, etc. and at a "reasonable price" is a real bottleneck as I see it but one way or the other, I will build the "Victor Slocum". I "sorta" like Robb White's approach to using the 18 hp Kubota engine but that engine is too much for the "V.S." if the engine has to be kept operating at or near its peak output. I'm sure a smaller diesel like that is available but I really would have to see Robb's application for the reverse feature before I'd use it......that "chirp" coming out of the engine box when you depress the rear of the engine 3/16"....would need to know a little more about the whole setup......I think I'll try to get in touch with Robb on this.