They look like light leaks to me.
The shutter curtain would be my first suspect, but I am not an expert.
The images of the camera itself show it is quite dirty and suffering from quite a bit of corrosion.
The dirt can be cleaned up and the curtain can be replaced but the corrosion looks nasty.
Looks like the former owner bashed it on a rock before he tossed it in the lake.
Before tackling the job of replacing the curtain I'd suggest you give it a thorough cleaning and replace all the light seals. See if that does the trick.
Honestly, better copies of an MX can be had if you intend to take lots of pictures.