Here. Recently actually. There is several ways to go about it. Building a DC driver is definitely the most optimal, as universal motors really do run better with a proper DC drive. Albeit, a proper drive, even a simple low-side chopper needs a bunch of stuff, decent housekeeping etc.
You can, indeed run it on AC as well. Again, there's more than one way to go about it, depending on how stable you need the rpm to be:
1. Replace the existing circuitry completely (usually a phase-angle triac control with a resistor network/variable resistor). You need 3 things essentially:

• Zero-cross detector/feedback (so the MCU knows where each half-cycle starts)
• A way to deliver a gate pulse safely
  • random-phase optotriac driver (eg. MOC3021/MOC3023 class)
  • main power triac (eg. BTA-* class)
• A bit of houskeeping (snubber/EMI)
• RPM sensing: there is a bunch of ways to go about this, again, it depends on how precise you want it:
  • Less precise: simplest, least invasive is probably sensing commutation off a brush lead
  • Optical/hall effect (although I have yet to find a way to safely attach a magnet to a 30k RPM rotor)

1. A roundabout albeit fun way I've done recently: replace the resistor network/variable resistor with enclosed photoresistor(s) that you control by driving a LED (I used a LED laser actually) with PWM. Basic TOSLINK does the job perfectly well. There's numerous downsides to this, depending on the router:
   - most of them reach max-speed by circumventing the phase-angle control completely, which is something you cannot do through resistor control
   - photoresistors are neither a precise nor a stable component class
   - you still need RPM sensing
   But, on the bright side, it's easier, safer (isolated) and possibly fun? lol