One Pole System Starter ?'s
The original one pole system starter was a 829527 which was discontinued and replaced by a 2 pole compliant starter 859252 (along with a cable and attachment 968048 and 945408).
Volvo Penta then discontinued the 859252 and replaced it with a 3581774 (and a cable and a nut, 968048 and 9994) which allows it to be used with your existing one pole system even though it is the same starter that is used in the 2 pole system.
The OEM starter 3581774 is pretty pricey however so what we often do is to replace it by an aftermarket starter made by a company called Arrowhead.
The Arrowhead starters are good, probably not as good as the OEM starter, but a whole bunch less expensive. You still need the two Volvo Penta special parts though 968048 and the 99094.