You can date the bike by looking at the date stamped on the outer shell of the SA 3-spd. hub. Here's a link to older SA hub service manuals, it's probably an AW hub, I've been through mine, they're really not that complicated, just make sure you keep careful track of and properly reinstalling the pawls and pawl springs:
http://www.users.globalnet.co.uk/~ha...samaintind.htm

I'd also recommend spraying some WD 40 or other light-med weight rust inhibitor into all the drain/vent holes in the small frame members, e.g. the chain and seat stays and lower braces.

Those look like they might be 451 rims. My 20 has the 406s. More tire choices are available for the 406 rims.

I geared mine down, from a 15T rear sprocket to a 19T, for better hill climbing performance.

Some links:
http://www.gaerlan.com/others1/ralei/ralei.htm
http://lennytaylor.freeyellow.com/Twenty.htm
http://www.users.globalnet.co.uk/~hadland/twenty.htm
http://www.sheldonbrown.com/raleigh-twenty.html
http://drumbent.com/folders.html