I need the key,washer and nut. Where can I buy them without buying the whole kit?
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.