Chains come in several different lengths and the number of links is usually indicated on the package.

If the chain was or wasn`t cut isn`t as important as if its a good fit. It should be safe to asume you can reuse that original chain, but you`ll need a quick connect link or a reinforced connecting pin specifically for that chain size and make. You can`t reinsert that rivet that was pushed out to break the chain.

If you don`t have the equipment - take it to a shop. It`ll be cheaper than buying the tool.