agree...you are lucky!
I took a few measurments with a caliper in case you wanted to try a local machine shop.
Bolt circle ~ 4.11"
bolt holes ~ 0.25" (counter sunk)
Inside diameter ~ 3.23"
Outside diameter ~ 4.09"
maximun diameter ~ 4.65"
thickness ~ 0.16"
PS... after some googling, I can't believe I can not find some "standard" as I have seen the same configuration used in many applications
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