Can I access data saved on iCloud from a dead Mac and download it to a new Mac?

You can sync any data from iCloud that you had synced with iCloud on the old Mac by turning on the same sync options in the new Mac.

You can view what data was synced by using an internet browser and logging into www.icloud.com

Data that can be synced includes iCloud Photos, iCloud Drive, iCloud Contacts, iCloud Notes, etc. The sync options are at Apple menu > System Preferences > Apple ID > iCloud.

Also see: Set up and use iCloud Photos - Apple Support

and

Set up iCloud Drive - Apple Support

and

Set up and use iCloud Contacts - Apple Support