From the ones I've seen, and like many products, the older units seem better made than the newer, so I think I'd fix it if I could. Suction is created by a leather gasket inside, which can dry out from long disuse, and can sometimes be revived with mineral oil, and sometimes replaced.
Before you disassemble, though, are you sure it's broken? Put water in the tank and start pumping. You may have to pump quite a while before water comes out. There could be a split in the water line somewhere so all you're drawing is air. I'd check the lines and connections carefully first.
If it is dead and you are unable to fix it, new hand pumps are readily available online or through RV parts stores. An electric on-demand water pump connected to a regular faucet might be a nice upgrade, though, if you plan to make much use of the onboard tank. You can buy single handle, cold-only faucets if your Scamp
does not already have one.
My hand pump does work, but getting enough water for anything more than brushing your teeth is a tedious process. We don't use our on-board water system. A 6-gallon jug with a spigot outside and bottled water inside works for us.