Zippkits has an ABS cowl that might work as it was made for the RCMK rear-exhaust engines. A back-halved Thunderboat cowl can also work. That's basically what I used on my JAE gas 'rigger running an RCMK R-254.
I ended up by using the "T-Rex" cowl from Tsunami38 on Jim's RC Boat Dock. I tried bracing the Zippkits cowl but couldn't get an adhesive to stick to the wood. Maybe I was using the wrong type of glue.
Not really as I had a rough time trying to get it. Even though that guy that makes them is just on the other side of the state, I had to go elsewhere to get it. Like an earlier posting said, maybe you can find a Thunderboat cowling and just use the part behind the driver's seat as that's all it is. Possibly someone you know has one where the forward part is damaged.