If you know where the original VHF was installed, look all around for the coax cable that connects to your new radio…
Should look like one of these provided by
@jssailem :
Looks like you already have power connection there in your photo.
It is hard to believe that the audio plug in your photo was somehow connected to your old radio as an antenna… if it was, looks like you have some extra work to do. But hopefully you can find the coax cable, hook it up and have a working VHF radio.
i have replaced several on different boats and left the old antenna and cabling in place.
One other word of wisdom…if you find the cable, hook everything up and still get bad reception or cannot transmit, the cable itself can degrade, chafe, short out and need to be replaced. Hopefully not, but since you don’t know too much about the history of the boat…who knows.
For going aloft, if necessary, I would try to hire a rigger or if you know people at the marina, maybe someone has a chair and is willing to go up the mast to help,you. You need at least 1 person to crank the winch. We send one of the “lighter” guys up (and is a pretty good mechanic, so once he gets up there, he can usually fix things). The other guys take turns on the winch.
Good luck on your new adventure. Always lots to learn about boat systems, hardware, sailing the boat, etc…
Cheers,
Greg