The Mullet SS is a 26" trials bike with an eccentric bottom bracket and disc brakes. The OLD is most probably std MTB=135mm.
Shimano Alfine will fit and give you 8 or 11 gears with a good chainline. Sturmey Archer X-RK8 is similar.
The frame seems to have a replaceable hanger but the one supplied has no hanger. Try swapping for a std gary fisher one.

The bike still lacks derailleur cable stops which are needed for high precision shifting. Hub gears use fully sheathed cables that plug directly into the hub.

The Mullet SS is such a unique bike that is is probably more valuable as a SS than geared.
There are plenty of 1x8/1x9 trials bikes around.