I can see the F for full advance and two dots (timing mark) with one dot on either side of the two dots (timing tolerance between the two cylinders) but I do not see a mark for top dead center. Shouldn't there be a mark for TDC for valve adjustment?
My 1978 R100/7 has the F-mark, followed by the S-mark with some lines, and then the OT mark. Try putting a straw in the open spark plug hole and turning the engine using the rear wheel. When the straw reaches the most outward point, you will be near OT. Find the point midway between when the piston is moving out (coming to TDC) and going in (after TDC). That will be OT. When you are rotating the engine, the marks will appear in the top of the window first and travel down. OT should be only a few degrees of rotation after you find the S-mark.
The '81-on bikes have a special new clutch-flywheel combination. Actually, the flywheel in that instance is called a "clutch carrier". But it will still have the appropriate marks.
Other ways to lighten the existing heavy flywheel consist mainly of taking it and drilling holes in it in a precise manner. Usually, though the outer edge remains the same with the usual marks. Here's a picture of a /5 lightened flywheel.
There are aftermarket flywheels that come lighter such as Bowman. A friend has a lightened flywheel in his R60/2 and much of the flywheel has been scalloped away only leaving enough around the edges for the timing marks. Of course, the /2 doesn't have an electric start for which you need teeth completely around the flywheel for the starter to engage.
If you do have some kind of after market flywheel, you'll need to precisely figure out where OT is. There is a device which screws into the spark plug hole with a dial indicator that monitors precisely the movement of the piston. Once you find OT, then knowing the exact diameter of the flywheel, you can determine how much distance around the circumference equates to 1 degree. There's information that says where the S-mark is located BTDC. IIRC, it's 9 degrees for the '77 models. In Jan 1978, it went to 6 degrees. You could then measure the distance along the circumference and make your mark. You could also use a degree wheel which needs to be firmly and accurately attached to the front of the crankshaft. Once OT is located, you can move the engine to find the appropriate degrees BTDC and make a make, usually with some white paint.
That makes sense. I have actually run across a lightened flywheel sort of as you describe. It's on a friends R60/2 and I believe it's called a Bowman flywheel. In viewing the flywheel through the hole, we would see nothing for the longest time and then a "tape" would show up with numbers ranging form say 35 degrees to TDC and then a few degrees after TDC. After that nothing. I think they milled off all except about 45 degrees on one side and the opposite 45 degrees, leaving "nothing" in the remaining 270 degrees. On the /2, there's no starter teeth, so they can actually get rid of quite a bit.