Need some help finding an old powermac

Any 8100 series, or the machine can be replaced with a 9100 series, to retain the same architecture, if you have hardware that requires NuBus connectivity.

FWIW, the 601 CPU is incredibly fragile, messing around with the heat sink should be done very gingerly, and should be done to anything still working now, just to keep it that way. The heat sink compound has surely dried out long ago and should be refreshed.

Which is why many of them have died off.

Once it quits, there's no coming back. It can be cleaned with acetone, near 100% alcohol, or similar, and just an 1/8th in. dot of new compound applied. Place the heat sink back on and snap down opposing corners and then the other two.

Sounds pretty specific for the film machine it's controlling.

I'd stick to a PowerPC of the same vintage. PowerPC machines are easily identified by the FOUR numbers in the model "name".

IE, 7100, 8100.

Personally, if you can't get another 8100, try for a 7100.

A "68K" machine whilst supporting NuBus cards may NOT run the program as expected. These have THREE numbers in the model name. IE Quadra 700, Centris 650. Older 68K machines like the Mac II series (IIcx, IIci, IIvi etc) have less chance of giving you a trouble free swap.

Bear in mind that even the 7100 like all these older machines WILL suffer from leaking surface-mount Electrolytic Capacitors which will corrode tracks.

With all that said, A Quadra 700 DOESN'T use those capacitors and will NEVER suffer from leaking capacitors on the motherboard. BUT - you'd need to test if the software will run on it. It DOES support NuBus (2 slots) and does have ADB, but space for only ONE hard drive (external SCSI is present) and NO Internal CD Drive.

You should also check what monitor connection the setup is presently using since the 8100 had a proprietary HDI-45 video connector AND an standard early Mac 15pin Video port.

The only PowerMacs with Nubus were the very first generation:
Power Macintosh 6100
Power Macintosh 7100
Power Macintosh 8100
Workgroup Server 9150

(And their variations, which might have different last two digits, or might be called 'Performa' with the same first two digits, such as the Performa 6116.)

Also note that the 61xx series may not have an available NuBus slot - they were slim form factor desktops with just a PDS (Processor Direct Slot,) that by default contained a PDS-to-NuBus riser, but many of them were sold with either the AV card or the DOS Compatibility card, which takes the place of the NuBus riser.

Your safest bet is to look for a 7100 or 8100 machine. (The 9150 is extremely hard to find - it is the same basic machine as the 8100, but in a taller tower, and sold exclusively as a server.)