Go fishing
Interesting dilemma . . .
The chain is not magnetic (or at least the one on my 150 isn't), and the fuel pickup is mounted low on the inboard SIDE of the tank, a short distance above the bottom of the tank, hence unusable fuel.
152s are slightly different, but unusable fuel is about the same.
To be safe, drain both tanks (remember you have a crossover tube), and go looking with a mirror and a flashlight. Use a "claw" tool to retrieve the chain. If you don't have one, you might use a straightened out clothes hangar, or any other long, stiff wire with a hook at the end.
Be careful not to drop anything into the tank, and remember that the unusable fuel is still sloshing around in there, so no smoking and no sparks! You can get some but not all of the remaining fuel out by removing the quick drains and lifting or depressing the wingtips.
Ground the airplane and the gas cans before draining fuel - this procedure is a very good source of static electricity, and is considerably more risky than fueling the aircraft in the first place. If you keep the fuel clean, when you find and retrieve the chain, you can put the fuel back in and save some money.
While it is doubtful the chain could clog the fuel pickup or fuel line, why take a chance?
Best Regards,
Echo Mike