Are there any aftermarket tachometers that fit in the stock dash hole?

Yes, it's 4.5" or 115mm opening. I have a VDO with a programmable shift light, chosen because the font is similar to stock gauges.

VDO Comp Eliminator. It comes with a programming tab that's removable. Not exactly a stock look but not bad and it works well. There are other styles if you prefer.

Or just use the stock teach and drive over the red line.

There's a post from a couple of years ago describing how a guy put a modern tach inside a 914 tach housing. I don't know how it turned out long-term but it looked pretty good.

Check out timothy_nd28 in the member vendors forum

http://www.914world.com/bbs2/index.php?showtopic=217100

Sadly the modern tach they sold isn't being sold anymore. It was a good answer, I did over 30 conversions with great results.

Why am I not able to find this at a retailer? Is it still being sold? Any idea what the VDO part number is?

Autometer monster fits, a hair loose, but 3 small blobs of RTV sticks it in good.
You lose the high beam and turn indicators.

Huh, I don't see it. Sorry, that was from ten years ago and VDO has apparently moved on. There are other options though, just look for 115mm/4.5" tachs with a switch for number of cylinders. And an LED shift light option is really handy, it's bright enough that you don't need to be looking at it to see it come on.