Canyon's mag looks like one of the recent manufacture .303 10-rd mags. Nothing wrong with that, other than looks, assuming it functions OK. However, such a beast is more suited to a plastic stocked hunting conversion, and his classic rifle deserves better.

Where to find 5-rd mags? An original Lee Speed 5-rd magazine will be practically impossible to find. A cavalry carbine magazine fits aesthetically but you'd have to lop off the mag loop and this would destroy a scarce item---I cannot recommend that. Besides, those are practically impossible to find too, and they deserve to be with a proper cav carb, not bubba'd.

Here are some options you might consider:

1) Get a milsurp 10-rd magazine. As seen in the attached photo of a similar sporter, it looks pretty cool. Customers could choose either a 5 or 10 round mag, so it is correct in appearance.

2) A few decades ago, an American company called Santa Fe imported surplus Lee Enfields (milsurp, not commercial sporters) and did a decent job of converting them to sporters. They had special 5-rd mags made in Japan. Below is a pic of these Santa Fe mags. These can be found if you look around. Be careful to get the ones made for the SMLE MkIII, not the No.4 rifle. They are not compatible.

3) I have seen some new-made mags that look like the one you bought, but in 5-rd size. These would not look out of place. In fact they look similar to the 5-rd mags found on some LSA Lee Speeds (see last pic below).

Good luck and congrats to your late grandfather for picking up this classic rifle. Kudos to you for choosing to preserve it.
...
First is a Lee Speed sporter original 5-rd magazine. These varied slightly in shape, but this one is typical. These would be very hard to find, but included here for reference.

Next is a 1920s BSA Lee Speed sporter with original 10-round magazine (identical to milsurp 10-round b/w a better finish):

Santa Fe magazines:

A 1920s LSA Lee Speed sporter with original 5-round magazine (note contour, not fish belly):

A newly made 5-round magzine I saw on GB for $25. I don't own one, so cannot vouch for its reliablity, etc. But it looks a bit similar to the one just above.