No fancy tools needed, nor even Harken's hallowed blue plastic capture tracks. However, BEFORE you remove the car from the track, build a 360 degree dam out of duct tape around the end of the track from where you intend to remove the car.
AND if you're feeling paranoid, have an assistant standing right next to you with a Shop-Vac, to assist in endeavoring to prevent bearing escapees.
Okay, I'm making up the Shop-Vac thing, I've never actually tried it, it just sounded cool in my head
Once everything's been removed, & hosed out, the new bearings standing by in a Ziploc or bowl.... Take some tooth paste, or shaving cream & lay a light coating into the recesses of the car where the bearings go.
Then put a dab or two on each bearing & insert them carefully into the car (while working over a bowl with a cloth in the bottom so the bearings don't bounce out when you drop them).
Now, once you're done re-installing the bearings, & the ends are on the car, slide it back onto the track.
Affix the end stops to the track, but don't pull off your duct tape dam yet.
Try rolling the car around a bit & test it for function. And yeah, I know, it'll be a little sticky at this point.
Assuming it checks out function wise, grab a garden hose & gently start to rinse the shaving cream/tooth paste out of the car. And if bearings don't flying helter skelter, just use a bit more water
pressure to fully rinse out your temporary "ball bearing adhesive".
Oh, & when choosing an "adhesive", keep in mind that shaving cream breaks down somewhat quickly with time. So, while it's easier to rinse out, you have less working time (AKA it's for "Pro's").
Once the car's fully tested, go have lunch, & think over the install. After which, have the village idiot come over & test out the car/bearings (to see if he can break it).
If all works well, THEN remove the duct tape, anti-escape barrier.