The exhaust from a Riviera is the same (except for length) and there are lots more Rivieras in salvage yards than Reattas.

Before someone jumps me for the above "same" statement. The systems are not identical.

The Riviera exhaust manifold, is the same. The pipe from the manifold to the muffler is the same but roughly 8" longer. You will need to shorten that pipe to make it work on the Reatta

You can use the Riviera muffler and tailpipe but the Riviera has just one outlet.

If it is a driver and it is a question of keeping it on the road (and not authenticity) then the Riviera exhaust will work fine.
Note: The '91 model has a larger pipe diameter, 2-1/2" versus 2-1/4, and may not interchange with earlier models without modification.
Thanks to Barney Eaton, member of the AACA Reatta forum, for this information.