not having access toa handset will not only make it hard, it will be impossible.
Everyone MUST HAVE access to the handset if they are going to replace a controller on these trucks, as the controller must be set to match the specs for the truck when you get it. You may get away with sending in the controller and waiting for the exact same controller to be returned ( 2 weeks about) as it will more than likely have the correct settings saved, no warranty that will be able to happen though, as the logics can be what fried in the controller.
A handset for these (part number 41-LXHS-00 from flight system industrial products; fsip dot biz) is not too expensive, and you will need the cable to go with the controller fitted into your machine to go with it.
you may want to see if there are any on e-bay, and also check with uniparts and TVH to see if you can save a dollar or 10. This handset is more expensive than what I consider "hand tools", which is what most of us are expected to have available at the job. it list price of about 440.00 USD, which, my guess is that it will be cheaper than 3 calls for the Yale dealer to send a tech out to plug in and set up the truck.
You should explain that _I_* guaranty, that they will save [anyone who owns or is responsible for 40 trucks with GE EV controls,] far more than it costs to own the handset, and if -you- own the handset, then it will go with you when you leave, even though his trucks will still be his.
you may find other information you desire at the FSIP, website also, so spend a few minutes looking around there. you may also want to include an e-mail address as part of your profile (not on the forum, the software will remove any properly formatted URL or E-mail address (it is a good thing; 'anti-phishing')
Motive Electric powered vehicles are not the same as Internal combustion vehicles, and as such, require some added safety and conceptual training that you may not have if you have not worked on these ttpes of units before, and while we can help some, I sure hope you don't get hurt before you get the training...
a few things to remember are to always disconnect the battery unless you NEED it plugged in, and always get the drive wheels off the ground before you do any work on it, as there are capacitors in some electric trucks that even with the battery disconnected for days, will have enough of a charge that if they discharge through the drive motor circuit, and the wheels are on the ground, the truck could/will jump far enough to crush a person standing behind (like the tech working on it) or in front, like the guy working near you.

*=I only speak for myself, around here, but at the least, I would bet every other person that makes his living working on electric motive powered vehicles would not argue with me about this.