That's the problem with the alloy blade plate ,if you got one cast up it would cost more than what the mower is worth.
If you just want a working mower Maurice ,the easiest fix may be to use a second hand Victa 4 blade mulching plate.
I've used the Victa 4 blade mulching plate on utilities but have bent the top 4 flutes on the plate so they match the blade angle that way they are not pushing air downward any more.
Yep an Alloy blade plate doesn't seem like a good idea Mf.
