Monel rivetts will cost you a little bit more but they're non reactive with the metals your putting them into so if you use them this wont happen again.
The state of that corrosion would make me get the mast thouroughly checked and if needed re-rivetted, the sidestays, forestay and bridle replaced.
It sounds a bit much to spend on an older boat but if you read my post I'm putting up now about me sidestay breakage this year it may encourage you.