The first thing is to strip pump and assess wear/damage.
Take care when holding pump ie dont damage by clamping hard in vice and you usually need impact driver on screws.
A hot air gun is useful on the screws housing to heat and aid screw removal.
I have found a common problem is damage to the rotor tips.
Large seals in rotor housing/body are not available from Honda, but there is someone selling aftermarkets ones,
http://www.ebay.co.uk/itm/Honda-cb750-sohc-oil-pump-oil-seal-repair-kit-/151706206379?pt=LH_DefaultDomain_3&hash=item23526500abSeal on rotor shaft is tricky to replace.
Stopper seal can be an issue.
Once dismantled you can see whether the rubbers parts are rock hard or still compliant.
Filter gauze on pick up is available from Honda/DSS and worth replacing if damaged on removal or rubber hardened.
Ideally replace rubber parts, but easier said than done
