Lockheed brake.

Commando or Navigator? Navigators had drum brakes.

Brake calipers are readily rebuildable - up to a point. But seals and maybe pistons will normally suffice.

Master cylinders are more problematic. A seized one often has a damaged bore. You could perhaps sleeve it to a smaller bore which might be even better.

But it depends what make you have. If OE Norton-Lockheed then parts are readily available from the usual suppliers. If it's AP Racing, aka Lockheed, then parts can be found.

Lots of options if you're prepared to go away from original.  Many people fit brake components salvaged from older (20+ years) Japanese bikes; readily available and work well. Calipers mount to the fork leg with an adaptor plate cut from alu plate. But might need a bigger disc and you'll probably want the matching handlebar switch too.

… currently supplied by Andover Norton, including period-correct versions of the calipers. See

https://andover-norton.co.uk/en/shop-drawings/34/ap-racing-products

Or just fit a 13mm master cylinder, standard hub, disc and caliper with braided hose - all easily available. You should then be able to make the usually 'last for ages' front disc look like it is actually wearing out.