Here's the skinny:

1 Set engine to 15 atdc

2 Lock the injection pump. (there is a 17 mm plug on the side of the pump. when removed you have access to the shaft that turns in the pump. at or near 15 atdc there is a small horizontal protrusion that can be held by a dowell with a slot cut into it. fabricate a tool to hold that protrusion centered to the access hole and make sure no matter what it doesn't move. if it does you're {more}screwed)

3 make (paint) a mark on the cam gear while at 15 atdc lining up with the tdc mark on the cam cap

4 remove the chain tensioner and cam gear. run a zip tie through the chain on the tensioner side and secure it so the chain isn't allowed to drop into the crankcase.

5 if your engine has a sheet metal "basket" held by 3 10mm bolts under the vacuum pump remove it.

6 remove the bolt holding the timer to the pump, note left hand threads

7 cut a soda can apart and get a good sheet of aluminum. slide that sheet between the chain and advancer and then slide the advancer out (ya know this might not be necessary with the cam gear out, if it gives ya any trouble use the soda can trick) care should be taken not to let the chain drop down into the pan where it can bunch up on the crank gear

8 reverse procedure to install taking note of the following: the left hand bolt should get loctite and on most models is a stretch bolt (should be replaced)

good luck