It sounds like a difficult change. The Auster type heel brakes are not powerful and fade quickly. Putting cable brakes on the top of the pedals is going to involve some interesting cable runs. However the marginal braking force on the cable brakes is enough to break the port side brake back plate some of the time - hence the routine inspections. Toebrakes would likely give more mechanical advantage and necessitate changing the back plates. End result is that instead of getting two good applications you may only get one!
You could change to hydraulic brakes - which I partly why I think this could get really complicated. I am aware of at least one Terrier aircraft which was fitted with them - back in the day when engineering calculations were not necessary. Given the Terrier's rather heavy back end it could work - from memory larger diameter sleeves were fitted to the axles and a set of American brakes and wheels fitted (possibly robbed from a C150). But to do it now would require some substantive design justification for the LAA to approve it.
Frankly a good set of drums and shoes gives 'adequate' braking for a Terrier, if only because it already has so much aerodynamic braking! Once the brakes on mine were sorted they only got used for manoeuvring.