How can I improve the brakes of my dutsun 1200. the brakes are currently manual with no booster and uses brake linings as opposed to brake discs

I'm assuming you have drum brakes, with brake linnings instead of disk brakes, on all four corners.
An easy upgrade would be fit the struts with disk brakes out of a 1200 coupe as far as i'm aware you would also need the master cylinder out of a coupe, which is needed for any disk brake convesion.
Alternatively there are many other struts out of bigger datsuns that will also fit that have bigger disk brakes like the sunny for example, these would require at least 13 inch rims.
Disks on the front is a good start but disks all round is better, on my rally car i have blubird trx rear disks but an upgrade to 14 inch rims is almost a nesessity, except for rare cases where some types of 13 inch rims would fit over.
As for boosters I'm not to sure but have heard that 180b ones have been used. I'm sure there is other options out there though.

Go to the "Main Menu" panel in the top left of screen.
Click "tech section"
Click on the photo of the front brake. It's the 5th item on the list. There is an article by Scot Davo that you should read.
Scot made a couple of small errors that should be noted.

He advises using the complete strut assembly from any 1200 coupe, or late ute, or ANY 120Y model.
The 120Y became Australian made after June 1976 & the brakes are diferent, so "UP TO" June 1976 is OK. These are the Japanese brakes. The later ones are PBR items.
He also fails to tell us that the smaller bore rear wheel cylinders wheel fron a disc braked donor car MUST be used, or the rear wheels will have an increased tendancy to lock up under heavy braking. This can be a real problem in the wet when the rear of your car overtakes the front.

If your car has the tandem master cylinder, you can use it, but it should be rebuilt as a disc brake model & that means removing the "residual pressure" valve for the original front brake circuit. The best bet is to overhaul it at the same time.

If you get yourself a workshop manual,... & READ IT,... you will have no problems.