Rerun, There are a number of simple things you can do to get the most out of your Norton and make it more reliable. These include uprated charging and ignition systems, suspension modifications, brakes etc etc. In terms of performance enhancements there are quicker camshafts, different compression ratios and all the usual tricks. Nortons are quite well catered for in this area because they are still campaigned enthusiastically in classic-racing. I suggest that you get the bike running first, you may well be pleasantly surprised with the standard performance and can then make an informed decission on what else is needed if anything and also have a reference point from which to judge the benefits of any modifications.
Regarding going the cruiser route, be very careful if you opt to put forward mounted pegs on it. The front down-tubes are thin-walled and many frames have been ruined when these get crushed with the fitment of these type of pegs simply bolted to the frame.
For a quick enhancement of your Norton knowledge you could do worse than trawl through the many pages of previous postings on this site, most of the problem areas are mentioned and lots of parts suppliers and workshops are recommended. I myself suggest that you get hold of the Clymer manual for Classic British bikes. It is being reprinted and contains technical and service information on BSA, Triumph and Norton in seperate sections and is one of the most udeful manuals I have in my library.
Try E-bay for a new tank, expect to pay $200= for a decent roadster tank, but a 'High Rider' tank will be less (smaller fuel capacity) and an 'Interstate' (much larger) will probably be more.
Good luck