To keep the stem straight for welding I found a tube in my junk metal pile that fit snugly inside.

No need for an analog speedo, the bicycle speedos work great. I have them on all my bikes.

I've used a bicycle speedo before as well, but I failed to buy one that is back-lit. I also struggled on my ATV, because the wire was so fine, it was easily damaged. The magnet was also a challenge. What kind of magnet did you use?

The ones I use aren't backlit either but I rarely ride after dark. On the rare occasion I do ride after dark I use a backlit GPS for a speedo. The sensor wire is indeed tiny and easy to damage so I zip tie it to the front brake hose or cable. I've also seen wireless units but haven't tried one. The magnet supplied with the bike speedo works fine. I goop it up with liquid electrical tape to keep it from moving.