[morzh]

For the transformer itself, I would (provided, I even consider dealing with a set like that) try to clean the surfaces and the bells (remove, de-rust, paint).
The rust on top of the core does not necessarily mean it is all the way inside.
By removing the bells, you will be able to see the conditions inside and judge as to whether the moisture/rust has penetrated deeper.
You could, after that, test the transformer and see if it functions, or it does not.


As for Hammond....considering the prices of their off-the-shelf products, I suspect their custom products of larger sizes will be not cheap at all.

I used to order custom from Heyboer; they had very reasonable prices, but they since stopped with custom ones.