Should'nt be a problem getting a suitable replacement capacitor.It prolly won't be exactly the same value,as they don't make them now,but an equivalent will do.
There will be several capacitors needing replacement by now,some of which could also cause humming.The main transformer can cause a low hum,but this is more a mechanical noise,and you can feel the vibration through the casework.
Well worth sorting out,lovely old kit,but you prolly need the help of an enthusiast,as to be fair,it's not really practical for a Dealer to be able to spend the necessary time.
Regards,
Nick