I think this conversion depends on where you live. Currently, a lot of niche market factories are producing small parts specifically engineered for fixed gear conversions.
I can see the value of the ghost chainring, but I think it's dangerous, and the ring would be dancing up and down all the time while you accelerate/deccelerate via the pedals, and bob violently on fast downhill, when it is very hard to catch up with the pedals, and when it's most dangerous if you have any surprise.
The magic gear has its value, and would give a very very nice setup: easy wheel assembly and reassembly, use a regular skewer (no bulky 15mm spanner to bring along), BUT I think it would last only for a few miles. Slack develops very fast, and you couldn't adjust it with the same chain (but honestly, I'm talking from theory, never used that either).
But I think the solutions that would give you more satisfaction are those that allow you to solidly compensate for chainslack, and most of them were already mentioned:
- Eccentric bottom bracket;
- Eccentric rear hub;
- Custom-made chain "tensioner".
That third option would be a metal pulley held in place below the chainstay by some solid structure rigidly attached (welded, clamped) to the chainstay, with some kind of adjustment (a slot would be perfect).
It would be necessary to attach it so solidly because, when deccelerating or skidding, the chain would compress this pulley against the chainstay with what might be a hard force.
Some frames might not be good candidates for doing this.
Anything you chose, keep in mind that:
- Fixies are DEFINITELY not toys. The chance you get killed (or kill yourself) on a fixie in traffic is much greater than on those free-ride bikes in steep trails;
- Fixies are so cheap that perhaps you assemble one as a second bike, and just "fix" your old bike back to what it was: geared, freewheel. Most likely a decent frame or even a whole bike wouldn't cost so much as you would spend with all these boutique components, which in the end are a bit like fancy workarounds.