For things like that, in the past I made a tool as shown here on the right:
However, I rarely look for some TDC mark, but do as breakwellmz says: feel through the plug hole. You have to take the plug out anyway. I use a very 'special' piece of wood for that, in order not to damage the thread or so, shown on the left.
2 more tips:
- This is a four-stroke (4T); the piston makes 4 'strokes' in each cycle (1.compression stroke: going up to TDC [Top Dead Centre] with the valves closed to compress the mixture, 2.power stroke: after the mixture combusts, still with the valves closed, it pushes the piston down to generate the power [= force x travel / time], 3. EX valve opens and the piston comes back up to TDC, to push the exhaust gasses out, and 4. EX closes & IN valve opens, to aspire a fresh mixture when the piston moves down again.) You want to adjust the valves when the piston is at the first TDC, the TDC at the end of the compression stroke. That's when they should have the prescribed clearance. You can feel where you are from the resistance given by the valve springs, or rather the lack thereof. When valves are being pushed open, the springs on that give resistance. At the right TDC there is no spring resistance, around the other one there is.
- I don't know how you plan to rotate the crankshaft to that TDC. What I normally do, with the bike jacked up & the rear wheel off the ground, is put it in gear and move the crankshaft using the rear wheel. Top gear is easiest. If the bike does not have a centre stand, and if you cannot jack it up safely, you can put it in gear and just move the crankshaft by pushing the bike forward or backward. But then it gets to be a bit difficult to do at the same time. Maybe behind the big slotted screw in the centre you'll find something to rotate the crankshaft with a key.
Good luck!