My donor's speedo was mechanically driven from the front wheel but as I'm not using a scooter front wheel I need to drive it from teh front wheel off a FWD car. I've taken the drive shafts apart and now have a pair of empty ends. One of them I've cut the "cup" off so it is a flat plate with a splined shaft and thread for the hub nut. This will hold the hub to the upright nicely. I'd like to do the same to the other side and weld a coupling on to it so that it takes the speedo pick up point. I have no idea what the drive shaft stub is made from. Is the metal likely to be a grade of steel I can weld to?

in short.. Yes

You can weld to it, but it will weaken it slightly but should be ok.
I sleeved my sierra shafts and welded sleeves ok for 18 months but then snapped one on start line at santapod, it was the weakest link.

Its only driving the speedo so torque isn't going to be the problem. Its a plan with no draw backs then.

I think driveshafts are made from spmething like EN16T but not hugely knowledgable in that area...

I cut and sleeved my Ford Puma driveshafts using a sleeve made from a lump of EN16T (tough stuff!), TIG'd it all together, and it's survived a Snetterton trackday and several hundred miles so far, so it should be ok to drive a speedo for you!

Ed.