Home-made 26" freestyle fixie bikes..........
 

Thinking of building a 26" fixed-gear freestyle bike using an old 90s Diamondback MTB frame, even if I have to braze in some track-style dropouts. This way I can have a light double-butted frame for cheap. It seems all the ready-made single-speed 26" bikes out there are either heavy or really expensive, so not for me. All I need to get me going is a fixie hub that is a nice fit in the Diamondback frame, and some track dropouts to braze in. Also since the frame is bare right now, I will need a nice light front fork to use with it. The rest will be easy.

Your fork will need to be a straight blade, fork. Pay attention if your donor frame originally had a suspension fork or not. If it did, you will need to find a suspension corrected, straight blade fork. By the time you have your correct fork and your track fork ends brazed in the frame, you're approaching the cost of a used purpose-built bike.

Some 80s ATBs have horizontal forward facing drop outs.
my RockHopper does and a HardRock that passed through my hands did.

and those are CHEAP now days.

of course track drop “backs” work better, but note you’d just be after wheels.