Any sealant that is not damaged by petrol will do the job, but may not be the best solution. I had the problem once with a Partner chainsaw: I bought it with the tank fuel-line seal missing. I went to a mower shop and the counter-jockey fished a generic line-and-seal out of a parts drawer and snapped it straight in. I had to buy a fuel pickup filter to match, but the result was as good as new.
If you have to use sealant, choose one that is rated for use immersed in petrol. Silicone RTV (room temperature vulcanising) is the obvious choice, but isn't very cheap. At the other extreme, ordinary epoxy will work for a year or two before the petrol breaks it down.
