I can recommend a local firm in my area, Google -
They specialise in stainless steel exhaust fabrication and alloy wheel restoration.
They will tackle anything though as they are an engineering firm.
Go to the engineering section of the website.
If you know want made, or if you have the old frame they will fix / replicate it.
Can vouch for their work having had a motorcycle exhaust made by them years ago.
I hear nothing but complimentary comments on their work in general .
I'm sure they would be willing to tackle your job.