Why not make a single seat version using something really obtainable as a powerplant? My specs are as follows for a low power single seater:
Tuned Briggs & Stratton 18hp v-twin engine, direct drive to 40x36 prop.
Ply fuselage, wooden/foam wings, fabric covering... or all metal.
Small baggage rack capable of 5~10kg weight.
Fuel tank for 3hr+ duration.
Flying weight ~120kg.
Powerplant would cost about 2k depending on modifications. I would recommend simply reducing the stock engine to its core and dispesing of any plastic coverings, fan cooling etc. This alone would bump up your power some, and apparently this and a later model carburettor (twin barrel) gives 21hp from the good old Briggs.
Fuselage would be fairly narrow, no more than say 24" wide at its widest and only as deep as you needed for structural and practical reasoning. The head and upper body would be enclosed in a streamlined cowl - you could make this multi-facetted or form a plug for making a single plexi piece.
Essentially you're looking at something between a Robinson-Rand KR1, Leon Davis DA-11, Mory Hummel Hummelbird, Fisher Flying Products Avenger, Team Mini-Max 1600R/1650R and the SD-1 Minisport. To be honest all apparently perform well and would be great to build from kit or plans, although only the SD-1 Minisport and DA-11 are designed to use the Briggs & Stratton engine.
All can be made for 15k or less by the homebuilder.
My faith in the Briggs is mainly because it a lightish engine thats proven to be reliable and has a large corporate source of parts that isn't going to go anywhere quickly. It also has a large aftermarket scene and is regularly in use for aviation. Its contruction also includes electric start, altenator for DC current and an... oh my GAWD... easily accessible oil filter! Its rpm are roughly the same as a VW conversion such as a Jabiru, Hummel 1/2 VW or Great Plains.
Cheers - boingk