E brake systems are all pretty basic- - - -either levers or cables or a combination of both that mechanically apply the rear brakes, or some type of system to hold the driveshaft and keep it from turning. Since you've still got about half of the system in place, adapting some sort of handle or pedal to fit inside the cab shouldn't be all that difficult. Just be sure whatever you use has enough mechanical advantage to put a sufficient amount of pull on the cables. An inadequate or unsafe e-brake system is probably worse than having none at all.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!