That is a question with a lot of answers. Old Britts of course has a whole bunch of information on the bikes, but like most it is missing the 69/70 blow-ups. Not to worry, about the only difference from the '71 is the oil tank, headlamp mount, center stand, side stand, breather assembly, harness, seat, I may have forgotten some things. One important thing is the front fender has shorter stays by 1" for a reason, if you get the longer stays, you may have a problem with steering under heavy braking because the fender will get caught in the 'S' exhaust pipes. The rear fender also has a notch for the tyre pump which I am finding is impossible to find, the fender, not the pump.
Depends if you want an original looking or just want to make it ride-able. I am trying to get mine looking as original as I can with some upgrades without going to whole different parts. You will have to look around for some of the original parts, Old Britts has some if you call them, they don't have a search feature for special 69/70 parts and they are real nice to deal with. I have had good luck with Mike at Walridge for frame, lever and rim parts, Norvil for engine parts, RGM for center stand headlamp shell and rim and some small stainless items. Amazingly you can buy stuff from UK including shipping cheaper than from the US. DomiRacer has some good deals, but most of their stuff is Oriental pattern and not always of good quality, they have a good price on the air filter and sump gaskets. Stay away from a place in Chicago and a guy named Marshall unless you go there personally and know what you are getting. Fair Spares does good upgrades to the gearbox and tach seals and has lots of parts, and SS handle bars. Rabers, British Only, Hawkshaw I have dealt with on small items.
I replaced most of my UNF bolts with SS from boltdepot.com. The Whitworth, CEI, etc. you will either have to have plated or buy special which is very pricey. Rocky Point has lots of that. I plated most of mine with Caswell's Copy-Chrome and so far it is holding up fine.
If you are looking for something specific, PM me, I will be glad to share my experiences. You can check out my 'Phoenix' thread too. I'm waiting on the head from MMW which should be a couple of weeks and then I can get serious.
A couple of others on the forum have '69'S', Nelson, sidriley, and possm, I'm sure there are more.