You can still get new 5 speed wheels and freewheels.

If your bike still has the original rear wheel, you might be able to just put on a 5 speed freewheel.

However, some fixed gear conversions involve moving the rim to the left by loosening the spokes on the right and tightening those on the left, because the wheel doesn't need to have enough space on the right for five sprockets. If you have the old wheel, and it was redished, then it would require some skillful work with a spoke wrench to get it back.

The cost of the freewheel, derailleur, cables, and levers might make another bike more worthwhile. You may also need a new axle or at least the spacer.