UKH

I've recently started using a pair of secondhand skis already mounted with dynafit st radicals. However I've noticed the brakes aren't very effective.

I think this is possibly for two reasons. First of all the camber of the skis seems to make the effective legnth of the brake arm quite short. And secondly it doesn't seem to take much pressure to push the brake arms in compared to my previous bindings (dynafit verticals).

Is it possible to change to brakes with a longer arm or stiffen them up somehow?

I don't know about changing the brakes, but I have chased a number of skis, one for a Km or so, when the brakes haven't been effective.  We also lost a ski in powder because brakes tend to drag the ski down.

So, consider ditching the brakes and using leashes.  Not without their issue too of course, but more reliable.    And your ski is lighter and less prone to clogging up with snow.

Leashes are great, if you have the habit of fumbling your skis when you try to put them on... Not so much if you fall.

Best case when ya fall is that the skis only whack ya in ze head a few times... But to be honest, that has never happened to me. I have, how ever broken 3 pairs of boots (or rather the leash clip fastening point) and a few leashes (the ones that go around the ankle, plus ripped a (half a ) pair of tele-binders from the ski...

The last one certainly was really fun, as it happened when I still had more than 700m of vert to ski back to the car on less than stellar snow.

As for the OP. I seem to recall ST has the option of changing brakes, but it might involve changing the whole base-plate... Which will be rather spendy and a lot of work, with not much benefit.