Adding this here in case it helps someone in the future.
It turns out that the clutch I need is not so common and finding one is not that simple. The specs: 15HP motor, 1" bore, 1/4 keyway, 4" pulley. I tried most of the major manufacturers sites, eBay, Amazon, etc. I did find one for $384 and a 10-day shipping policy from the distributor of NORAM clutches, but the price seemed unreasonable to me.
So I disassembled the clutch with pullers and eventually a sawzall to get the seized bearing out. The springs and brake shoes in the clutch look new, no rust, and appear to me to work as designed. (First time taking apart one of these.) The bearing was press-fit but then held by a welded cup on both sides, despite being also held by a bolt on the end of the shaft. The experience did not make me nostalgic for all my time spent in Chinese machine factories over the years - I ground the weld out and then politely and calmly got the bearing out.
The bearing is a 35mm x 50mm x 20mm, and I found one in stock on Amazon for $25.00. It arrives sometime between today and Christmas, and I'll press it in and report back to see if it made a difference.
Thank you to everyone who has helped.