It sounds like the threads in the muffler are stripped out so that the smaller correct size screws in the plate won’t thread into place. Best repair would be to weld shut the holes in the muffler, then redrill and tap for the smaller correct size screws. Or get some screw in thread repair from Ace Hardware that will match the original smaller screws. I can’t remember if there is supposed to be a gasket under the plate but it isn’t really necessary. I hope you have enough aluminum in the muffler to accept the thread repair. If not, you might have to drill out the plate and use larger screws. Tough call.

Please do not use that plate. The J-25’s all used a pipe that fits into the muffler and it has a 90 round exit. If you use that cover plate, drill holes in it so the exhaust can escape. Otherwise you are closing off the exhaust and the motor will not run. Art DeKalb sells a repop of the exhaust snout for the J-25’s. The later J’s did use a cover plate but that is because the exhaust housing had side holes drilled in it so the exhaust could escape. I think the models were the J-75 and J-80’s.

http://www.precisionservoutboard.com/ This is Art Dekalbs web site. You can contact him thru there.

Even if it’s not listed,ask Art about the part.He has some castings that are not listed.