I have never seen an arrangement like that on the boom of a Super Maramu.
The original metal frame with 4 bolts through it to the gearbox is necessary to counter act the torque produced by the motor gearbox combination, especially when you are tightening the outhaul to flatten the foot of the main, plastic doesn't make it.
Sorry, but I would recommend that the boom be returned to the original configuration with the complete metal frame.
WRT to getting the shaft out ...that's a common problem, because of the salt water ingress through the top gearbox seal.
Maybe take the boom off and take it to an engineering shop that can press it out, and then clean off the shaft so it fits easily and coat it with Never Seize when reinstalling it.
BUT I really think you need to have that gearbox support rebuilt as per the original.
In any event your new paint will need redoing.