I think features you should consider when selecting a replacement forklift seat should include:
Operator restraint design - there are different philosophies relating to assisting the forklift operator to ride a truck down. Most popular are hip restraints or shoulder restraints. Maintaining the OEM style would be a good idea.
Hole mounting patterns - some seats have a wide range of mounting hole patterns, matching those with your current seat will make safe replacement much easier.
Special features - such as folding backs or tilting bases to facilitate hood opening with LP tanks or restrictions to engine/battery compartment access.
Seat cover material - if it's going outdoors, vinyl's the way to go, strictly indoors cloth may be more comfortable and resistant to rips.