Dougie have a word with RDG they do a nice heavy 8" flywheel, its not on their site or e-bay store at the moment but they had quite afew earlier in the year.
This is one on my Cameron pump

And one for the current Tidman engine, they are actually 2" wide to start with

TS and RC in Germany have some good flywheels as do M&M in the states. Also worth looking at Engineers Emporium who list quite a few in their paper catalogue but not on their site, also Atelier
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