Digikey's 3.9uF cap worked great. Last time C1 blew, I just swapped analog boards. This time I repaired the original analog board by replacing C1 with the Digikey cap, swapped the boards, and it fired right up, working perfectly.

Thanks guys!

I use this Mac Plus to run Altoclock off a bootable floppy. Once it boots and I launch Altoclock, the floppy spins down and there's no moving parts from that point on. It's a completely noise-free digital clock for my kids' room. Yes, it's overkill to use a perfectly working 4MB Mac Plus computer as a digital clock, but it's cool and very retro, and frankly it's the best use I've been able to come up with for it.

Once again, thanks to you guys for the tip on the capacitor!