If your speedo (that is, the needle) is jumpy, I'd suggest you take a look at the speedo cable and the two angled gears first before scrapping a working speedo. The plastic angle gears are prone to brake and the typical symptom before breaking completely is jumpy behavior. My speedo was jumpy when I bought the bike, and then stopped working at all, with a little help from me not mounting the angle gear correctly on the speedo, when the angled gear on the engine sheared. Ordered a new one and now it's working.
BTW, the reason I was fiddling with it was that my kilometer counter had stopped rotating, while trip continued to function. Reason was some missing teeth on a plastic gear inside. Found a cheap used Veglia Borletti from an old Ducati SS on german ebay which from pictured looked identical to the MG one (except for graphics). It was identical, so I just adjusted the kilometer counter, mounted the MG graphics and needle, and remounted the ducati speedo in the MG housing (which was slightly different). Works as a charm now.
/Anders