there is a bloke called Kev Armstrong here in the UK that has been upspecing a B&S twin with excellant results. ideal affordable candidate for a minimax i believe.
heres a copy paste from a uk microlight site post:
Briggs & Stratton V-Twin tuning parts and reduction drive now available in UK
For anyone wishing to build their own tuned briggs vanguard we can supply the tuning parts and reduction drive. Listed below are the prices for both a low power application and the full power as per Kevin's build:
28-30HP build requirements (all in stock)
HC head Gaskets - £30 pair
High flow filter, billet manifold & throttle linkage - £135
Rev kit incl ignition advance key - £85
Stainless Steel Exhaust Valves - £66 pair
Roller rockers - £345
Total: - £661
Reduction drive (1.8:1). - £600 (complete with all hardware and belt ready to fit in either orientation, prop high or prop low)… (stock expected next few days)
Total £1261
Some people have opted not to use the roller rockers for the low power build hence saving £345. They are however required for the high power build.
Additional requirements for 38-40HP build including the roller rockers:
High lift Camshaft (40hp) & followers - £185
10:1 piston and rod set - £420
High torque solenoid shift starter - £150
30mm twin choke Carburettor upgrade - £220
Total cost as per Kev Armstrong build: £2236
Other optional extras can also be supplied:
Re-usable Copper head gaskets W/sealing rings - £80 full set (available in different thicknesses 0.6mm – 1.0mm – 1.2mm)
Stainless steel inlet valves - £66 pair
Billet aluminium adjustable flywheel - £325 (no charging facilty)
Billet finned rocker covers - £160 set (special order)
Titanium big valve ported heads - £950 (special order)
All the above prices are applicable to retail sales and are inclusive of VAT, we keep many popular items in stock, any out of stock and special order parts have a minimum lead time of two weeks.
CAD drawings and DXF file for flywheel machining will be supplied free with kits as will any relevant advice, if required we should soon be able to offer a flywheel lightening service on an exchange basis but costs for this service have yet to be established. Exhaust manufacture has so far been on an experimental by application basis but we hope to standardise a design and offer a kit as soon as possible. We do however already have CAD files for general design and cad/ dxf files to simplify exhaust manifold plate manufacture.
For orders please contact Brian at Fallows Motors 07925 027138.