I have a Federal Arms HK91Clone. Back in 2000 that was the only choice I had if I wanted a HK91 clone or never be able to buy another AW(Assualt Waepon) in California.

The Federal Arms clone is junk and the receiver is now broken. Being a registered AW in California the serial number is priceless to me in allowing me to own a AW.

Does anyone know if it is legal to get a new receiver to replace my damaged receiver?

Gmotoring

It is possible, with ATF permission, to replace a receiver with one of the same serial number.  However, it is the original manufacturer that must do this.  You need to contact your manufacturer.

I think it is pretty limited as to what you can and can not do with registered AWMB uns.  I feel this was California's intent to not allow transfers or replacement of registered AWB guns.
They probably have statisics where there will be no "assult weapons" buy a certain date.  Maybe 50 years?  Eventually ALL registered guns will have to be sold out of state as people expire.