Get one of these HDI Marine Stainless replacement elbows. It comes with all the gaskets. I did that on my J/109 and it fixed my exhaust flow. Additionally, if your engine is raw water cooled as I suspect it is, look at my old J/30 Rhapsody blog about rebuilding the engine to unplug the water passages.

https://hdimarine.net/product/qm-stainless-steel-exhaust-kit/

[Linked Image from hdimarine.net]