Do you have a picture of the old unit, &/or a model number for the gas control valve the thermocouple is connected to? Is it something unusual?
Thermcouples for standing pilot gas appliances have been fairly standardized for long time. There are
generic replacement kits that can be trimmed to length,
or replacement modules that come in different lengths. Often old-school hardware stores will have an assortment of lengths in stock to choose from, and with the old one in hand you can eyeball the connections and length to figure out if it'll fit. The big box stores might have a few too, possibly in the "water heater repair parts" section.
Is this beast old enough to collect Social Security, or carry an AARP card, perhaps? At some point they're not worth fixing. If you think yours is nearing that point, it's worth
monitoring it's fuel use to get a handle on the actual design heat load, so you don't end up with something sub-optimally oversized for the load. Older furnaces were usually on the order 2x oversized for the original heat load, before the house got updated with better insulation, sealed up drafts, & better windows then end up 3-5x oversized for the "after-updates" load. With hot air furnaces oversize factors that big not particularly an efficiency problem (the way it would be for a cast iron boiler), but right-sizing it for the actual load is much better for comfort, running long duty cycles when it's actually cold out rather than 8 minutes on with temperature overshoot, followed by 25-35 minutes off with a big undershoot (chill) in the rooms at the end of the duct runs.
Slow steady heat might seem like it's "struggling to keep up", and makes it harder to use a deep overnight temperature setback strategy during cold snaps, but keeping the oversize factor bounded is key to maximal comfort, comfort most homes don't have. ASHRAE recommends an oversizing factor of 1.4x the load at the 99% (not 99.6%) local outside design temperature, which is enough to cover you through typical cold snaps or even a Polar Vortex event, but not so oversized that it's uncomfortable and unduly noisy. AFUE testing presumes a 1.7x oversize factor, which can be big enough to cover coldest hours of the coldest days since the last ice age, temperatures you'll never actually see.
