Cracked Standpipe / Replace 48 Sq Ft DE Filter?

The standpipe assembly on my American Products 48 ft^2 DE filter cracked. I don't know how but it's over 20 years old and the deck over it fell down (and was reconstructed) five years ago: it could have started then. The standpipe assembly seems unavailable - I've searched a few websites and most say some variant on "no longer available." If anyone has ideas on how to source a replacement, I'd be thrilled to hear about it.

Is there a replacement filter that will connect directly to the existing plumbing or am I looking at a major repair with bunch of plumbing work?

How many GPM am I looking for?

Thanks for your help.