The connectors are designed to be changed easily. Common name for them is "Q-C", as in "Quick Change". Pull the four screws, and it will 'snap' in and out.
RF Parts has them in all types. There are doubtless many other sources, like Surplus Sales of Nebraska.
Seems to me the "UHF" female (SO-239) type is around sixteen bucks.
Leaving it in line is NOT supposed to affect the antenna system. The Bird is a short section of precision 50-ohm air-insulated coax. Rigid coax.
The common "UHF"-style coax connectors have more power loss in them than the Bird's coax section.
But not nearly enough to measure outside of a calibration laboratory.
Haven't compared prices in years, but if you're only interested in 27 MHz, you have the choice of two versions of the "element" (slug). Type "H" covers 2-30 MHz. Type "A" covers 25-60 MHz. Used to be that the equivalent "A" slug would be cheaper than the "H" slug with the same Wattage range.
I may be out of date on that, but it never hurts to compare, if that's how you'll be using it, on 27 MHz only.
Slugs made by Dielectric Communications and by Coaxial Dynamics will normally work, if they're meant for that type meter.
Bear in mind that not every slug that will fit in the hole is right for a model 43 meter. Bird makes meters for industrial applications that take more current to drive them. Normally a Bird-branded slug will have the power range, followed by ONE (only) letter that indicates the frequency range. A type "1000H" would show 1000 Watts full-scale between 2 and 30 MHz. A "500A" would be a 500-Watt slug for 25-60 MHz.
ANY additional numbers or letters following that first letter at the end COULD mean it won't be compatible with your model 43.
Slugs made by another source won't use this numbering scheme, far as I know. Got their own system of catalog numbers.
73