Chris,
Sorry to hear you have had a bad experience with your speedo. Most owners take one look inside the speedo head and are completely baffled by it's workings - but if you've got an hour to spare to check out it's operation, you will find that it is not that complicated. It consists mainly of an escapement (much like a watch) which trips the needle back - this works in opposition to the drive from the wheel - and the needle records the difference. A 'twitchy' needle is a characteristic of the chronometric unit - that is the reason for the 1mm driving pin fitting in a 3mm hole in the driving gear - the gear can then twitch backwards and forwards without transmitting that movement to the needle - instead, it gradually nudges the needle one way or the other. The actual speed recorded is governed by the number of extra weights added to the balance wheel - adding more weight slows the balance wheel down, thus giving a higher reading.
Being purely mechanical, most problems with old units are down to dried up oil causing the delicate components to malfunction. Careful application of WD40 will usually get things working again - but be sure to relubricate before refitting.
With regard to the smeared markings on the face - replacement faces are available from several reputable sellers at most autojumbles.
Regards,
Eddie.