Michael offers excellent advice, he's recently "been there and done that".
I would like to offer a little more. First of all I would not even think about replacing the head gasket without doing a valve job with new valve guides AND ensuring that there is no corrosion damage to the head. My M103 head exhibited the same symptoms as yours and it turned out to be the water jacket corroded through from lack of regular coolant replacement. My machinest welded it up.
When laying the camshaft back in place, it is important to start all bolts fingertight, then gradually tighten them all down a little at a time, if you don't do this, you could break the cam or strip bolt holes.
The tool for the seal that Michael correctly told you that you don't need, is substituted for by using a fingernail around the seal as you push the upper timing cover rearward into place. You are seeing that the seal does not "fold".
The importance of following the tensioner instructions cannot be overstressed. If you do not, you can break something expensive because you will be putting it back together with tension. Push the ratcheting plunger all the way through after removing the tensioner assembly, then start it from the outside, don't push it any further than just starting it into place. Upon startup, oil pressure will ratchet it into place.
You can do it either way, but I left the injection system in place, rather than pulling it off with the head. It worked very well for me, I had to remove and replace a few of the manifold bolts from underneath, but it was no problem. It was preferable for me to prevent ruiining some of the vacuum and electrical connections on the injection system.
Use the sealant that Michael mentioned on the upper timing cover. It is slick and allows you to push the timing cover rearward without disturbing the USeal below the cover.
Best of luck and keep us informed,