Q: Can I access the drive on a Power Macintosh 7200/75 as an external drive?

Wow, haven't seen one of those in a while. Nice Mac for the time but hopelessly outdated these days, unless you need access to some classic (System 7, 8 9) MacOS for, well... reasons.

These old Macs are not going to yield to much in the way if all you have is a new Mac and the CPU itself.

The internal drive is SCSI making getting data off of it a bit problematic.

Your best bet might be trying to find an external drive case that supports SCSI drives with a USB case. Do these even exist anymore? New ones, possible, maybe, not sure. Used ones, more likely on eBay and such.

Failing that you may have to source a keyboard, mouse and monitor to investigate further.

If there is a Mac user group in your area someone there might be willing to assist you with the necessary equipment (loan/rent) or get the data off the drive for you.

The PowerMac 7200 should an Ethernet port, so if you have the relevant adaptor for your MacBook Air (either Thunderbolt or USB), you can connect the two.

You'll then have to Start sharing in the File Sharing Control Panel on the PowerMac, and then if I remember correctly, set up Users or guest as you desire. Then you'll have to select the folders or disks you want to share, and under File > Get Info > Sharing, you can share that resource.

But this will entail having a mouse, keyboard and monitor. You may be able to source some of these cheaply in tech junk shops or ebay.