Check with partsetal (Carl Heller) here on the forum. He may be able give some info on a great replacement if his vendor has rads for "Little Birds" too. I purchased mine for my Squarebird through him and couldn't be happier. Thought about the Aluminum route but would have needed to have a custom one built that included a transmission cooler $$$$ and the info Dave mentioned on heat exchange.
Went with the copper/brass as original and it's super nice.
Check this post for info and pics....