The JMJ pickup is larger than the US Mustangs.
2 reviews here are from JMJ owners, one doesn’t mention any figment issues, one mentions a few screw holes were slightly off: Invalid Link Removed
You can get a guard custom cut from a number of different places, but it will require you to send in your original or a tracing and will likely cost you a lot more than the above. There are also some on EBay that claim fitment on a JMJ or MIJ Mustang, but I’d expect you might find some minor screw alignment issues as above. FWIW, I have a JMJ original pickup and pickguard mounted on a Squier Mustang body and there was only a single screwhole of that didn’t line up suggesting there is a decent amount of consistency between different Mustang templates as long as you choose the right size of pickup hole (JMJ and MIJ Mustangs use the same larger pickup size, everything else AFAIK uses the smaller size)