You might reserach this company. http://culvert-rehab.com
I'm sure it is not cheap but might be the best and most economic long term fix. Before I did anything I would go in and determine how much rout is present before ruling out just patching it as you are thanking about doing. I think there are other companies that come it and spray a concrete liner in your existing pipe but I can't seem to find a link to them. I had a new all aluminum drop structure of simialr size installed in a new pond in 2007 and it cost 8900$ installed. It might be possible to get a track hole in, build a coffer dam around the existing pipe, remove it and install all new with out draining the entire pond allowing the fish to stay in place.


The road goes on forever and the party nevers end...............................................