I am most likely repeating myself.
There are lots of investment casting businesses in the US. Even your dentist does it. Take a reasonably good set of sideframes and make a non-shrinking mold. Then make waxes and have new sideframes in lifetime brass.
Jay C is a participant on the 2-rail side, and is an expert in this area - all you need is a complete set of relatively undamaged sideframes and some postage - and a reasonable pile of $ for the work. Ask him nicely.
Jay helped me with the nose of my Erie-built Diesel, and it turned out beautifully.
Lionel could do this for you - for a price. Maybe they could split the cost?
This is not rocket science - it is a technique thousands of years old. Lionel wouldn't have to lift a finger - except for packing up a set for reproduction.
There are cheaper ways - pewter, or even plastic. But brass is elegant, and does not have zinc pest.