If you want to go aftermarket, any 2 1/16" gauges will work in place of the indicator lamps, it's just a question of how much work you want to do. If you want them to look factory, you'll have to remove the trim rings and glass, and perform significant surgery on the inner plastic structure to make them fit. Then you'll be on the hook for wiring everything (though Painless makes a nice harness) because you won't be able to use the original printed circuit. The real trick IMO is finding an aftermarket speedo and tach that are the right size. I finally settled on VDO, as they had 4" units that fit better than the Auto Meter's (and others') 3 5/8" or 5" units.
Food for thought: