I don't think there are any other modern source ports for Marathon out there.
You can turn off Mouse Acceleration in the preferences menu.
Found a fix for FOV on the PC Gaming Wiki
Field of view (FOV) Make FOV file Go to Scripts in the installation folder. Make a new file called FOV.mml Add the following line: <marathon><view><fov normal="120" extra="160" tunnel="70" /></view></marathon> Replace 120 with whatever FOV you want. Extra and tunnel are FOV settings used for the extravision power-up and the tunnel-vision mode. Extra should be higher than your normal FOV while tunnel should be lower.
Looks like you're stuck with 30f/s though unless you can find a way to uncap it in the source port. Maybe there is a patch or something out there.
Honestly though its a super old game, there is absolutely no reason for it to run faster than 30fps. It's not like it's an online RTS and you are playing that the professional competitive level.
I think it'd work with Leela, but you just need a better text-to-speech - maybe a paid/premium one, unfortunately. I like Amy under the Plus Voices here for instance.
Not sure if this has everything, but this might interest you:
Note not all of them are new, but plenty of sellers sell new ones and are restocked frequently.