No and no and maybe no., the B25 crank is a shell bearing big end with alloy rod of the wrong length, B40 is roller bearing with steel rod. C15 is smaller than the B40 head and the mounting studs are further apart. The B25 gearbox has normal road ratios, the B40M has wide radio's for off road, so take your pick.
Thanks for the info, I was dubious about the cylinder head anyway. Does this mean I can at least use the flywheels & shafts, and i would prefer the road gear ratios anyway. I am actually trying to locate a complete or near complete motor (B40GA would be nice) in preference, but this was offered to me.
Does this mean I can at least use the flywheels & shafts
B40 shafts are hardened steel, B25 are unhardened and keyways and splines wear out, and the B25 shafts are integral with the big end in single piece not separate unlike B40. The construction is completely different due to big end difference.
Other than the gearbox nothing is usable, buy the gearbox if they will split or get another one.