Since I’ve got about 7.5k mostly off-road miles on the old T4, the steering rack is pretty sloppy! Installed a new one pretty easily though… Had to use a new steering u-joint since the spline on the Joyner rack is a bit smaller/metric. Used Latest Rage part number AC425152 and a 5/8”- 36 spline - 3/4 double D shaft.
Mine does a little freewheeling at times. Revving the engine will get the rpms up enough to activate the engine brake.
I don't know any other way to activate it when it doesn't kick in automatically. So if you've tried that already, then I'd say that you have an issue.
The 610 was a pretty popular model. A used one will turn up, given enough time and effort.
$150 bucks seems fairly expensive though. I'd expect something like that to be in the $50.00 range.
I see lots of semi local craigslist, and fb marketplace adds, that're looking for salvage/junk models. I'm assuming that it's a wrecking yard for atvs. You should try to contact one of those guys.