Leave the cambelt on, remove the spark plug on cyl. 1, put your thumb over the hole and turn the crank until you feel pressure (don't forget the piston can rise on the exhaust stroke), stick a *long* straw in the hole, turn the crank (clockwise) until the straw hits its peak.
Not 100% accurate, but it will let you know roughly where TDC is and hopefully line up some other markings for you.
TDC is meant to be stamped on the flywheel, but this relies on the flywheel never having been changed. It can be spotted through the inspection cover in the bell housing.
But TBH, you don't need to set the engine to TDC when changing tappets. Just mark the crank in relation to the block and aux shaft, the cam in relation to the head and mark the belt in relation to the cam, aux and crank pulleys (obv. with distinctive marks). Then just line up the marks when you put it back together. Plus, it will also ensure the belt goes on the same way round
