Welcome!
I picked up a cloud badge Slingerland marching snare for a mere $40. It has some extra holes and was painted very, very poorly. I love the sound, and I am going to make it usable for play...Unless I am making a huge mistake by doing anything other than playing it, I plan on doing the following:
Old (20th century) marching drums don't generally have much monetary value at this point in time, and one that's been drilled and painted probably never will. So it's kinda hard to make a "huge mistake". But I'll chime in with a little (hopefully helpful) info and point out some potential gotchas that you may want to be aware of...
The strainer doesn't do much; in fact, you can't switch it on or off. I suppose that snare strainers for marching snares don't need to be on or off.
A strainer with throwoff was an optional upgrade on Slingerland marching drums before the late 1950s. Throwoffs have been standard since then.
1) installing a natural-finish Trick aluminium strainer... I was going to use something like a Ludwig P-85 strainer, but the Trick is just so much of a nicer strainer. If I am going to bugger it up with a modern strainer, I may as well go for broke.
Personally, if I was going to use a modern strainer, I'd use one that didn't require any drilling. It just feels so wrong to put holes in an old drum. I think a Ludwig P86 would look nice on an old marching drum and it matches your original strainer's 2.5" screw spacing.
3) re-wrap in something funky, like neon pink satin flame
First of all... UGGGGHHHH!!! Now that I've gotten that out of the way...
Wrapping the drum might make it really difficult to find heads that'll fit.
Since the late '50s, coinciding with the introduction of plastic heads, drum shells have been built slightly under their listed size. If you measure a modern 14" shell, you'll find that it's more like 13 7/8" across. If you measure your old marching drum, you'll find that it's exactly 14 (or 15 or 16) inches. Most modern heads, being made to fit modern shells, are going to be a tight fit on such an old drum if they'll fit at all. And if you re-wrap it, you're adding another 1/16" or so to the drum's diameter, making it even less likely you'll find heads that fit.
Aquarian American Vintage heads are the only current off-the-shelf plastic heads that'll fit my 1950s Slingerland drums, including a marching snare. If I wrapped that drum, not even the American Vintage heads would fit.
4) getting a lower-profile batter-side hoop. I don't want to whack the bejeezers out of the taller wooden marching hoop.
If your drum is like mine, you might need to replace the wooden hoops in order to go with modern heads anyway. The collar depth on an American Vintage head is too shallow for my old marching snare. When that head's on the drum, the tension rods don't quite reach the lugs.
Of course, if you replace the hoop, you'll probably have to replace the tension rods as well since they'll be about 1.5" too long.
...but you'd be able to sell the claws from your marching snare to someone who needs replacement claws for an old Slingerland bass drum.
And finally, just because we like to share pics, I've attached shots of my old Slingerland marching snare as received last year and as it looks today. It's a 1953-54 Standard King, which I coincidentally snagged for the same price you paid for your drum.
