All the 337's for the mods on my airplane had been turned in,it was all signed off & legal -- just some of the STC paperwork for the engine conversion turned up missing by the time I bought it. I'm sure that with a good IA (who's not afraid of a little paperwork), a cooperative FAA inspector, some time, & maybe a little money, most paperwork issues can be ironed out.
Bob Williams of Udall KS sold three basic mods for Cessna as far as I know-- a STOL kit (leading edge cuff), tailwheel conversion for C150/152, and engine upgrades for 150/152 & 170/172/175. I have one of their sales brochure from years ago-- the engine kit was marketed as "Avcon", the t/w & STOL kits under both the Avcon & Bush Conversions name. The name "Martin Aviation" is also on the STOL kit brochure.
FWIW the "Texas Taildragger" C150/150 kit uses an Avcon engine mount & muffler. Harry Dellicker of DelAir (Porterville CA) now holds the STC's for the three "Texas" C150 conversions: tailwheel gear, 150/180hp engine, & 40 gallon fuel tanks. But I don't believe he is marketing them at this time, he's mainly into upgrades for the 170/172/175 series cessna's.