Hi Jeff,
Here is a picture which shows the boiler side of the #90 safety valve, and also the boiler bushing:
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My only Empire experience has been with the B30 and B31 models, but I think there are enough similarities to the 90 to lend my comments some merit.
Your safety valve looks nearly identical in form to the B30/31, with a slightly different body. The valve stem is actually a 6-32 x 1" round-head brass machine screw, along with two 6-32 brass hex nuts. Stainless would also work. Here is a pic of a B30 stack, with integral safety valve, showing the component parts:
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The picture shows three different valve washers (the part that fits under the head of the screw), two rubber, and one a stainless/silicone rubber combination. The original ones I've seen seem to be some kind of black rubber with a cloth fiber reinforcement. The picture also shows two different springs: one is an original brass spring, and the other is a stainless one from McMaster-Carr, which can be trimmed to length as required.
I like to use the stainless/silicone rubber washer, as it seems tailor made for the application. It is stainless, which won't corrode, and the silicone rubber is rated to 400 degress F. Also, the rubber extends up thru the hole to provide a seal around the bolt. The hole is 1/8" dia. so it's a good fit with the 6-32 bolt (.136" nom. dia.):
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This washer is #91195A120 from McMaster-Carr, and costs $3.65. If you don't have an account with them, I have some extras, just send PM.
By the way, the #90 also had a stack attached to the safety valve, but like Jensen stacks, they are almost always missing. It is likewise unsual for the whistle to survive intact, like yours. You're fortunate in this regard.
Paula