Where to find opel corsa timing marks on crank and cam pulleys to replace cambelt

the cam turns at 1/2 distance. It take 2 full turns of motor to return cam to the timing marks. have you changed timing belts, they do stretch.

timing marks are on the cam belt wheels and on the oil pump and camshaft housing. tension of the cam belt get adjusted with the water pump, replace water pump at the same time to save to do the job twice, because water pump leaks a lot of times a few 1000k's after cambelt has been replaced.

If you set the engine to Top Dead Centre before you started it shouldn't be out of position or at least not by much.
You will need two M6 bolts.
The crank pulley has a notch on the edge that will line up with a marker on the engine block at the 12 o'clock position.
When the camshaft lines up there will be a bolt hole for an M6 bolt to lock the cam in position. It will be roughly at the seven o'clock position.
The injector pump lines up using the same method as the Camshaft. The bolt will fit at roughly the 4 o'clock position.
I have included an image as a rough guide.

Do you mean valve timing or are you past that and need to do your ignition timing? What year Opel? Did you line up your timing marks when replacing the timing chain? I am including a link to a video that may be applicable.

must be on right timing in all pulleys and gears on mark then remove it

Use the timing marks on the damper for the crank and bring it to 0 on the cam there are marks that correspond with a mark on the head. Look on autozone dot com and put in the year make and model then look at the free repair guides.

You need to find the timing marks on both crank and cam pulleys, set it, then take of the belt ane dont let any pulley move! Put on new belt and set the tension of the belt.

This is the best I could find for you .. I am unfortunately able to verify the info because I have no personal experience with this car or its engine ...

There should be marks stamped into the cam and crank gears that need to line up with corresponding marks on the backing plate before you remove the cambelt.

If there are no markings, you need to make your own.

Rotate crank until at TDC (reinstall the crank drivebelt pulley loosely and front cover to do this, then take them off again)

Once at TDC, use correction fluid and mark the crank position twice - once on the crank gear itself and a corresponding mark on the backing plate.

Ditto for the cam gear.

Make sure these marks are aligned when cambelt is replaced, turn engine one full revolution when new cambelt is on to double check marks line up where they should and you're good to go.

Sometimes the cam isn't marked but the crank gear is. In that case line up the crank gear and use the above step to mark the cam gear.

Be precise as the cam gear will most likely move when you take the cambelt off due to tension from the cam on the rockers

Hope this helps :)