850 Spider starter

[mach@domain.elided]

Hi

Lately my 850 Spider has been making a strange sound on starting up. It
fires up, but then there is a short grinding noise like the starter has
not disengaged. Kind of like the sound you get when you try to start an
already idling engine....:)
How serious is this problem and will it cause any permanent damage ? Can
anyone recommend a reliable CHEAP source of new 850 starters ? Mine is
the 903cc block.

Thanks and regards

Jason 
'72 850 Spider