Peter, Brian,

thanks for the bright thoughts. Think I know what caused the problem. Per
the plans my engine requires about 1.5" long spacers between the mount's
attach points and the engine itself. This creates an extra bending moment
which may have triggered the failure.

The mount is made of 3/4" 4130 tube with 0.035" wall thickness.

My current idea of repair is to weld a short round bar into the remaining
tube and so rebuilding the attach points. Maybe I'll shorten the spacers a
bit and bring some ballast further forward instead (battery).