MMP https://megamoto.co/ would likely be the best source for OEM MM parts. MMP is the result of GPS buying MM's parts inventory when MM went out of business.
Let's get practical...
According to the specs: You need a piece of 5mm x 5mm key stock that you can cut to the 62mm length. You need a 16mm flat washer, I doubt the 30mm OD is critical. You need a 16mm 1.5 thread pitch nut. It's listed as a flanged nylon-locking nut but in the screenshot (from a MM video) it doesn't look like it's flanged; I see a Nyloc nut and a washer. You don't need a flanged nut when using a washer, a locking nut over a thickish washer should be fine. In lieu of a Nyloc nut, you could use a regular nut and Loctite (clean the shaft and nut threads well before applying). For most uses, I would use blue Loctite but I might use red on that (may need to use a heat gun to remove the nut next time). If you have someplace that specializes in fasteners nearby, they should have what you need. Otherwise, a local Home Depot may have something that will work. I've done nearly all of my shopping online for several years, so I don't know how good the selection of metric fasteners would be in hardware stores these days. It wasn't great several years ago but I imagine it has improved.
The 5mm key is a pain when it comes time to changing sprockets...available sprockets are keyed in 3/16. Anyway, m16x1.5 are hard to find locally. I ordered a bunch of stainless thin profile nuts from eBay. I needed thin in order to align everything properly and still have full thread engagement on the nut. Stainless because of the higher tensile strength which helps with the thin design. So yah check eBay Goodluck.
5mm = 0.19685", 3/16" = 0.1875", 0.19685 - 0.1875 = 0.00935", So, basically 0.009" (9/1000") difference. In order of ease, you could: (1) Use a 3/16" key and not worry about the excess clearance/slop in the keyseat (not recommended). (2) Cut a shim from a 0.009" feeler gauge to use in the keyseat with a 3/16" key. (3) Make a stepped key, removing 0.009 +/- from one half of one side of a 5mm key, could do it with a Dremel and/or a file (you could remove 0.0045" +/- from each side but that just seems like more work). (4) Open up the sprocket keyway to accept a 5mm key.
Yep. That’s exactly what I did. I’ve done both the make my own stepped key and feeler gauges for shims. Both worked fine.
What really sucked today was the chines tav jackshaft. It’s also metric..16mm. The kits come with the metric sized drivens but if you use a comet or order a Chinese driven by itself they are 5/8. I had to turn down the shaft by hand and it wasn’t fun.