Again, I'd recco rebuilding the A4 but you really can repower with a
diesel for less than 6K. I just looked on TADiesel web site and they
have a used 24 hp (Perkins I think) for $3950 guaranteed to work. It
has a new control panel too.
I repowered my 28' S2 going from a 6.5 hp Yanmar to a 13 hp Yanmar with
no problems. I also thought it'd be a drop in replacement but it
wasn't. I had top cut down the engine stringers but othetr than that
it went well and wasn't too hard.