My '81 R65 came with the same "customized" seat latch and lock as yours! I scored a replacement lock box, the internals and a new lock and key from Re-Psycle BMW
http://re-psycle.com/. They are in Ohio. Mark and Marco are good sources for used BMW bits. Expect to pay about 50% of list unless it's something made from
Unobtainium.
Once the seat is open, you'll likely be able to see a wire "bail" with a short, heavy rubber hose on it. Roll this bail aft, disconnect the fuel line and pull the tank back -initially- then lift the front off its rubber pad mounts and pull it off up/forward. Really simple... with the seat open. Just don't drop that expensive fuel tank.
Check with Re-psycle for an ignition switch. Your're likely going to have to buy a new -as in $$$- fuel cap if you want a keyed cap. Start saving now. I bought a cap from them, sans key. Same goes for the fork lock. Unless you have a serious locksmith in your family, I see you carrying four different keys.
I have only one key for my '81 and that's the seat lock. No lock on the fuel cap, forks or ignition... that's kinda hidden. I often leave my empty "Uncle Mike's" pistol holster clipped on the bars. Curious types wonder, "Is he packin' heat?" Nobody has taken the holster, either! Well, this is Texas.
Suggest you spring for a Clymer manual to go with the Haynes. Both offer info not always covered in the other! Go figure. Also start tagging the many BMW tech resource sites on your compuKer as "favorites". Start with Snowbum's
http://bmwmotorcycletech.info/ Disregard the racoon he wears on his head. He really does offer a lot of quality info. Plan on using up 2-3 days just wading through everything.
Forget the dual plug mod. You already have your hands full. Just get it running properly so you'll have a baseline to work from.
Monte