That engine should have the small ears on the case. Any Fox .35 muffler will fit that engine. The problem with the small ears are that they break. They don't just break from crash damage but also vibration. This can be resolved by using steel brake line JB welded to the side of the cylinder.  The newer current  blimp style works, but it's pretty much a useless fishing weight. The new muffler will quiet the engine considerably but robs a lot of power and also retains too much heat. Cutting off the stinger on the end and allowing the muffler to flow as freely as possible is wisely suggested. Many don't like the fact of cutting things off, but this muffler will many times interfere with landing gear applications and removing the end is the cure as it serves a dual purpose.  The best option is to find a older 1974 era Fox muffler. Finding these are a bit tough but they can be found.  This muffler totally envelops the exhaust stack and is a direct fit for the Fox .35. The sides of this muffler can easily be tapped to accept two screws for a radiator hose clamp. The muffler can now be used to fit ANY Fox .35 including older wide bypass non eared Fox's.  Do a shotgun blast of the muffler and take out the guts in their entirety. Open the rear hole as large as physically possible. Screw or pop rivet the rear end cap on. I prefer the pop rivet otherwise the endcap will just fall out and you will never find it.

Randy Smith can also provide you with a far superior working tongue muffler to the stock mufflers that Fox provided. Randy's muffler will work on your engine with no modifications other than opening the holes up slightly. I usually open them to 7/64" and only add more holes if overheating becomes an issue. Here's a picture of what I feel was one of the best Fox mufflers. This is mounted on a 1980's Fox.35. https://www.facebook.com/PhillyFliersCL/photos/a.515143271889145.1073741831.513140418756097/673288052741332/?type=1&theater  This particular muffler was made available with the front end in different lengths one being long, and one short. I have to use a CB Tatone spinner backplate as a spacer to allow for prop clearance due to the longer nose. Ken

The 25FP muffler won't fit a Fox 35, it's about an 1/8" narrower than the Fox. 

The OS 35/40FP (RC os FP-S), LA 40/46, 25FSR-32F, Magnum 36, etc muffler will all fit the small ear Fox 35s.  But unless it's an aftermarket muffler for one of those engines, it's probably too heavy to consider.  There are a ton of aftermarket tongue mufflers that work well.  The stock Fox mufflers aren't worth the trouble either, since in stock form they're garbage and they tend to go for big money on Ebay.