I agree with Mike - adding turn signals is easy but trying to modify an OIF harness to fit your bike will be much more difficult (btw, it's '71 that turn signsals were first fitted, '70 is still pre-OIF and pretty much the same as '69).
Fwiw, I added turn signals to my '69 Triumph T100 and the following is all it takes. BSA's and Triumphs used the same wire colour codes from long before '69 and, '71-on, the same electrical components. British Wiring
does the wire and terminals, any of the usual parts suppliers will do you standard '71-on components. Or you might be able to buy a kit of components, wire and terminals, although you probably won't get any choices about the components or wire colours.
1. Mounting the indicator relay on the T100 was relatively wasy, 'cos Triumph never changed the frame, so it was just a matter of seeing where they mounted it '71-on (rear stud of the battery
carrier, above the mudguard) and doing the same on my '69. The standard relay needs power when the ignition key switch is 'on', so find a White wire (if you're using standard Lucas
wire colour codes) nearby and link from that.
2. The other relay terminal is connected with a Light Green/Brown wire to the handlebar switch.
3. For the handlebar switch, I used the cluster first used on the T160 and then by the Co-op '76-'78, most easily available as the Sparx SPX007
. This has the advantage that it's a simple bolt-on replacement for the '69-standard horn and dipswitch; you don't even have to change the clutch lever.
For the indicators, it'll connect with the Light Green/Brown wire from the indicator relay inside the headlamp shell and it has the Green/Red (left indicators) and Green/White (right indicators) also for connection inside the headlamp shell, you just need to run the relevant wires to the rear indicators' positions.
Finally with the SPX007, you also get a headlamp flasher.
4. Indicators. Dunno what you're thinking but I didn't fancy the size and expense of Lucas
or pattern ones. I went for these
at the front ('cos the black bodies blended with the black headlamp brackets) and these
at the rear, 'cos the polished alloy bodies blended with the chrome grabrail. Despite the apparently-different makers, the indicators are exactly
the same, and I fitted them all with tinted lenses, so I don't have the glaring orange either. I appreciate this retailer's based in GB, but they sell millions of 'go-faster stripes' for Jap bikes, Hinckleys and Hardlys so I'm sure there's something similar in the US; also, as the indicator maker(s) are almost certainly based in the Far East, you could well find 'em in a US 'go-faster stripes' retailer too.
5. Finally, don't do what BSA and Triumph did when they fitted indicators originally, and rely on the mounting to be the earth/ground path.
Whatever indicators you decide on, if they don't have an earth/ground cable running from the bulb holder, make one up at least with a ring terminal to go under the mounting nut, with the other end connected into the loom's earth/ground (Red) wires.