T&T: Onan exhaust elbow

[Bob]

I need to replace the cast wet exhaust elbow on top backside of my Onan MDJF
Gen Set because it is rusted through and leaks water.  I asked Onan where I
could get one and they say they are no longer in stock nor being made and
suggested to have one fabricated.   Part number in the parts catalogue is:
No. 155C1059, Ref No. 25 on page 40.

This must be a part that is in demand as I would think rust in this area must
be common.  Can anyone suggest where I could get this part or have one
fabricated?  Is it possible to make one out of fiberglass?

I will have to have it shipped from the US to Japan so, a west coast supplier
would be best... but if it is hard to find a replacement, then I will consider
anywhere.

Thanks for your help on this one.