I have been told repeatedly that a handlebar MC is the biggest single improvement, quickest fix, for the lack of brakes in a single Ate set up. Yes it costs something, not sure. But if you want a cheaper fix them the stainless steel hose will give you some improvement. If you want a cheaper fix than that replace the stretchy cable from the lever to the under tank MC. (admittedly only a tad of improvement if any)
There may be some improvement from new pads but I can't tell you which ones work best.
I've always thought a different rotor would be enough but I haven't had the moola to try my theory.
It is often found that a correct set up of the stock system will work better than what most riders are piloting. The swinging caliper system of the Ates on mid '70's Airheads have a specific adjustment to effect proper performance. You'll find info in your manual.
Or trade it for a Honda.