Unless you want an 100 % original car upgrading to an alternator is a very good idea (IMHO). Much more reliable and more output, also at tick-over.
If you are concerned about the looks there are alternators looking like an old dynamo. More expensive though. There is a good Lucas workshop manual available as a PDF on the internet:
http://www.gomog.com/allmorgan/LucasFault.pdfHave a look at the chapter "Dynamos and Control boxes"
One problem is the mechanical regulator for the old dynamos. It needs adjustments and often the contacts suffers from corrosion. Electronic regulators for alternators are MUCH more reliable. Inside the dynamo the brushes have to carry the full output current wheras the brushes in the alternator only carries the current for magnetisation, lasting longer.
