Are HUD systems available for light GA aircraft?
It looks like there have been several attempts at coming up with viable HUDs for light singles but none have, well, taken off (business jets are a different story):


The VirtualHUD company mentioned in this article doesn't exist any more
The G-HULP exists and you can buy it but I couldn't find any useful information about it or even the manufacturer's website


Since there's really nothing much out there, it suggests that HUDs for light singles are too expensive for typical pilots and/or just not useful.

Cost is always relative, but the standalone G-HULP costs USD 6,000 and that has to be compared to other, potentially more useful things like avgas (i.e. flying time!), avionics upgrades, ADS-B or just general maintenance.

But even if a GA pilot has nothing better to spend his money on, it isn't clear that HUDs are actually as useful as you might think. This article looks at a new Honeywell system for business jets that overlays a HUD view on the PFD, and they compared the landing precision of 'heads-up' (traditional HUD) vs. 'heads-down' (HUD on the PFD) pilots:


  A chart that showed landing points at the end of approaches flown by
  pilots using the two systems showed scattershot points with no
  apparent bias toward either system. Both systems allowed their pilots
  to fly very accurate approaches.
  
  This result, Ververs says, surprised the pilots in the study who had a
  lot of HUD time going into the testing. They had all assumed, she
  says, that the HUD would be the better technology. As far as the
  testing experience could show, it wasn’t better [...]


Of course that's only one specific testing scenario, but it's a very critical one because landings are when you really need to see and react quickly. Personally, I'm not convinced that focusing my eyes on the HUD for airspeed and then through the HUD to the runway would be any quicker or easier than just flicking my eyes between the airspeed indicator and the runway centerline. (Jan Hudec made the comment that focusing shouldn't be an issue with a "good" HUD, so this may not be an issue at all, at least in theory.)

And there are probably other issues with a HUD too, like making an instrument scan more complex; making the pilot focus his vision on the HUD rather than into the distance; or just being a general distraction.

There is an interesting alternative concept to a fixed HUD, by the way: wearable tech. Whether it's any more successful remains to be seen.