GSX400F starter problem

[stuartadair]

Evening everyone,
I've got a 1982 GSX400F restoration project which has stood for about 3 years.

When I press the starter button the starter motor turns very slowly, as if the battery were flat. Battery is fully charged and have run the starter with jump leads off a car directly onto the connections and it behaves the same way.

Took the starter to bits and found 2 of the 4 magnets had small pieces missing off the ends and 1 magnet come completely loose. I stuck the loose one back on and cleaned everything up.
Put power through the motor off the bike and it spins great but back on the bike it still wont turn over fast enough to start the engine.

I know there is something called a starter clutch fitted but not sure what it does? Do you think I should replace starter motor or investigate this sterter clutch ?
Cheers
Stu