I can't say all parts are a good idea as some are not as good a quality as OEM but a starter and alternator is fine to use aftermarket. Do not use an electrical automotive part. They are not ignition proof. I used a Mercury starter on my last boat with a Volvo 5.7. Just make sure it's the correct mounting style so bring the old starter with you to match. I am sure there are more variations of alternators so you will have to cross reference it.
I have also had ok luck with Sierra brand parts.
Again, some parts are just not as good as OEM like exhaust parts but others are fine. I had a sierra starter on the same Volvo 5.7 and it worked fine but was not painted like the original or the Mercury replacement. I guess paint was the reason Volvo wanted twice the price.
