R61 most likely is a varistor. This component is only used for dissipating overvoltage spikes on the mains input, and it degrades while doing that. If it dissipated a lot of overvoltage enery, it might start to burn (as it did in your supply). If the component is missing, the only thing you lose is resilience against overvoltage spikes. If the supply runs instable, you likely have another fault that will not be fixed by installing a new varistor at R61. Any sufficiently beefy varistor for 275VAC/400VDC can be used to restore surge protection.

Be careful: That varistor is connected directly to live and neutral of your mains line, possibly with a fuse before it. That's a dangerous area to work in, and I myself do not know what safety certifications are required on a varistor to be deemed acceptable by your local (or any other) regulations, but there likely are some requirements. You likely can get over 1000 watts of power at that location without any safety device (like a fuse or GFCI) interrupting the circuit, so if anything goes wrong, there is a fire hazard.

That's some great info—thanks! I'll see if I can find any info on regulations.