We have assemblies at work on our trains which are dissimilar metals and we use a non-chromate jointing compound between them to prevent electrolytic corrosion - i'm surprised the MGR never did originally.
In the case of steel vs aluminium, the ally is the sacrificial anode and so will dissolve and leave the steel untouched. I'd suggest probably getting a replacement crash beam and then using the compound between them.
I have seen in the past, barrier tape between mating faces, so you could use duct tape or similar.
https://shop4fasteners.co.uk/duralac...iABEgKaRPD_BwE