Series 1 dash lights weren't terribly bright to start with, but in any case you should be able to read the speedo and the gauges in the dark without any difficulty. Mind you, the brightest potentiometer setting is next to off.
As with any old neglected car, it is quite possible that the bulbs blackened out themselves (if both of them still light up), or are dusty or both. If you are itching, you could remove the speedo, inspect and clean up things (mainly the inner side of the glass), even try to find a couple of brighter bulbs to fit.
I have seen that you've tried to remove the speedo cluster and what is left to do, is to disconnect the speedo cable. It's not too difficult, you have to remove the driver's glovebox and try to reach the cable extended securing nut. A probably failed description, but at least i've tried. I think that the owners handbook has some figures that will help. If you manage to undo this, the speedo cluster will pull out, but you still have to disconnect the cables from behind to remove it completely. Refitting is indeed the opposite procedure in theory, but in practice it is a pain to connect properly the cable to the speedo.
Others prefer to remove the dash top instead, but since i 've never tried this method, i cannot really comment on this.
