I just wind out the whole tensioner cover/retainer enough to be able to get the gear back onto the end of the cam - ie. at the start of the job loosen the cover/retainer a few turns, then when it comes time to put the cam gear back onto the cam, wind the cover out enough to get it on. That cover/retainer has to be wound out an awful long way before it comes off, but really try to only take it out just far enough to get the gear on with quite some effort (including reaching down with a long screwdriver or rod of some sort to push the tensioner plunger in) - because if you let the sleeve in the tensioner come out far enough to extend beyond the pin in the plunger, it can get damaged when you try to wind the cover/retainer back in - the tip off that you're in strife is if the cover/retainer seems to hang up or come upon a resistance while you're winding it in - if you continue you'll break things.
Yes, I've done that too - and my fix was to chuck out the tensioner locking sleeve part (that's what broke) and just make up a little gizmo to keep the spring centred on the end of the cover/retainer. That tensioner locking setup is ridiculously complicated for it's purpose - which is unnecessary anyway as the spring is not very strong (it's only purpose is to ensure there's at least a little tension on the chain until oil pressure reaches it).
Early model cars have an aluminium washer for sealing the cover/retainer and later ones have a big O ring which will be pretty much unrecogniseable as it'll have moulded into a sort of conical shape - point is, don't go trying to gouge out the "build up" that seems to be evident under the mating face of the cover - that could be the O ring! (I know because that's what I did - bugger!!).
For access, let go the power steering belt then undo the steering pump mounting bracket bolts at the front of the head and the nut on the rear of the pump and lay it aside. The correct tool for undoing the cover/retainer is a sort of circular gizmo with two pins (or a C spanner type thing), but I just use a pin punch to whack the cover around. I guess you could stick two pin punches or drill bits into the holes and lever between the two to rotate the cover etc.
If the cover/retainer leaks, then you'll need to wind it out a bit again, thoroughly clean the area and then progressively apply black silicon sealer around the full circumference of the mating face before tightening it back down snug.
On a tangent - the warning about having to have the tensioner locked before removing the cover/retainer else injury be caused is crap - the spring is NOT that strong! Don't pull it right off though unless you're prepared to make up one of those tools, because to re-install it safely (without breaking it in the process) you'll need to compress and lock it then release it again with the tool once the cover is back on.
Also, don't fret too much about letting the chain go slack after pulling the gear off the cam - just ensure there's a cable tie to hold the chain on the gear (so you don't lose the timing) and then just let it sit where it wants - the chain won't fall off anything lower down.
When it comes to putting the gear back onto the cam, I use a 7/8 socket on the crank pulley bolt to turn the chain/gear slightly as required to line up with the slot (ie. with the gear partially over the spiggot on the end of the cam). You'll have all sorts of problems turning the cam with the rocker gear bolted on (subtle way of saying it's damned near impossible).
Always use a new cam gear bolt + always ensure the thread in the end of the cam is clean and dry + always use a drop of Locktite 262 Super Stud Lock in addition to the locking compound already on the new bolt + always torque the bolt up properly to 50Nm (NOT 50 FOOT POUNDS!!!) - I think that's about 37 ft/lb but check that if you're going to use an imperial wrench.
If fitting another cam it's always a good idea to "chase" the threaded hole with the old cam gear bolt to ensure that the bolt will run freely in far enough - when I fitted my Crow cam first time around I ended up with the gear vibrating loose which caused the little pin to break (luckily I discovered it before it all came apart). After fitting a new gear, it came loose again!! - I checked the threads in the bolt hole in the end of the cam and found that they had a build up of some hard substance at exactly the right depth to make the bolt come tight at a point where it wasn't pulled down properly on the gear itself!