Is 16mmx5mm such a unusual dimension for that?
I believe so, yes.
Unless you happen across any originals I think you'll struggle to find any accessories for this saw, but the good news is that you can easily make these and any other aids for your bandsaw that you might need. There's quite a tradition of this as you'll see the more you view and read about bandsaws (similar to how crosscut sleds for table saws are almost always user-made).
I strongly recommend that topping that list of things to make for your bandsaw should be a couple of push sticks. Literally the first things you should make once you have your saw up and running.
Unfortunately there's the usual maker's dilemma: you could do with the tool to make the tool. Without a push stick to saw out the typical shapes of user-made push sticks your fingers are closer to the moving saw blade than you should really be comfortable with, however there's a simple solution — you can use a pencil with an eraser on its end as a rudimentary push stick:
Or would it be feasible to get a cheap miter gauge from Amazon or eBay and replace the gliding steel part with a 16mm one?
Note: while direct shopping queries are off-topic for this SE I can suggest that you look on AliExpress to more directly buy the type of products — and possibly the exact same products — you'll find for sale on both Amazon and eBay, sometimes at substantially lower prices and often with free shipping.
So anyway, yes. If you hadn't asked this was one of the solutions I would have suggested anyway.
However while you can use a mitre gauge on a bandsaw, from what I've seen their use is far less common than it would be on either a table saw or a router table, which are more frequently called upon to do accurate finishing cuts and milling operations at various angles. By comparison squaring cuts are far more likely to be what you'll want to do on your bandsaw (after resawing and other ripping operations, and curve-cutting of course, which are the bread and butter of bandsaws).
Replacement runner material
For the replacement runners you can use a wood, high-density plastic (cutoffs from white cutting boards can work well), or from solid aluminium if you can find it in the right dimensions. But honestly it's not necessary to use metal, since wooden runners can last for many years even in a busy workshop.
Oak, maple, hickory, beech, ash, chestnut and hornbeam would all be suitable. Bamboo would also be excellent, even if it's not technically wood :-) Although you might be best favouring solid wood both plywood and MDF can work and are widely used for this purpose.
Since you mentioned a fence, as I say in the Comments these are independent of the slots for adjustability both for distance from blade as well as angle (to account for drift, if necessary).
There are a great many DIY fence designs out there and most woodworking magazines have featured at least a couple over the last few decades, although designs for them go back far longer.
Such fences can be very basic:
Popular Mechanics, July 1967
Or elaborate and fully featured:
Originally appeared in American Woodworker, November 2005, now available to view on the Popular Woodworking site