How do I clean out a lawnmower fuel line? I think it was fouled with rain water

I would say get all the liquid out and then let it sit out somewhere with everything opened up for a few hours to dry out. Pull the spark plug so the cylinder can air too.

I wouldn't use alcohol to clean it, you risk drying out seals that way.

I'd start by removing the spark plug and spraying the tip with a carburetor cleaner or starting fluid, then let dry or blow it with compressed air--or both.

Flush with straight gas. From the ones I've seen there usually aren't lines, the carburetor is siphon-fed and primed with a priming bulb. Rinse the tank with fuel, empty it, press primer bulb until it's cleared out. Attempt to start the motor 6-12 times to flush out what remains, that should be enough to empty it. Replace spark plug, refill fuel tank, prime, and attempt to start.

I haven't had to do a lot of work on lawnmowers, just cars and outboard boat motors. This is what I would start with, though.

Unfortunately I have a lot of experience with this.

You're going to have to remove your carburetor. Remove your air filter assembly. Disconnect the fuel line and let it drain out so your gas tank is empty. There will be a couple of control rods and springs connected to the carb. Take a picture of them before removing for later.

The carburetor will be connected to the block by probably two bolts. Take them out and the carb.

Unscrew the fuel bowl from the bottom of the carb. Probably 4 Philips head screws. Dump any fluid out and clean the whole thing with carb cleaner. Reassemble. Refill your tank with non-ethanol gas. Never use anything else where fuel may sit for more than a couple days.

It's not as hard as it sounds, even for us old fogies.