For what it is worth - - - - and not knowing your circumstances, budget, etc. - - -
Given the current cost of batteries, I'd recommend battery maintainers.
(And, no, I have no connection to the battery or maintainer businesses.)
It sounds like you may not use this equipment frequently. If you have the time to try it, you might put a battery maintainer on each battery for a couple of weeks and give it a try.
I was skeptical at first, but I've become converted. I've got maintainers on 6 pieces of equipment now. A couple of the batteries are in the 10-year range and still seem to be functioning well - - - knock on wood. I think this is particularly important for equipment that doesn't get used frequently, as the batteries slowly discharge and get sulphated, especially in winter.
I'm sure there are a lot of varied preferences out there, but I now use the NOCO Genius 1A. About $30 at Amazon. Inexpensive compared to a new battery and/or the inconvenience of equipment that doesn't want to start when you need it.
The maintainers typically don't like to work on a fully discharged battery, but even the low amperage (inexpensive) models will eventually top up a partially discharged battery if you give then enough time. And the de-sulphating mode may only kick in after the battery is fully charged. And the de-sulphating process may take a while.
So, without knowing more of your situation, I'd offer this.
1.) Disconnect both batteries and load test them individually.
2.) If they are good or 'marginal', check out the wiring, connections, starter motor, lubricants, etc.
3.) If they are sub-marginal or bad, you should probably replace both of them.
4.) Or, if they are marginal or even sub-marginal and you have the time available, you could try to resurrect them by fully charging them (individually) and them putting them on maintainers for a couple/few weeks. The cost of the maintainers is less than the cost of a new battery.
5.) Even if you replace the batteries, you might want to keep the old batteries and try to resurrect them with maintainers. You might actually end up with a couple of decent spare batteries.
NOTE: As someone else mentioned, you should probably avoid mixing old and new batteries in a system. An old/marginal battery will act like a parasite, always drawing down the new battery and degrading it more quickly.
Hang in there.