I'm looking for a library similar to this bitcoin one but that works with scrypt based coins.
The use of scrypt vs SHA256 has an impact on proof of work but no impact on the signature scheme or transaction structures. The only thing that should impact you when using this library is the address prefix that differs with altcoins.
The standard address prefix for Bitcoin is 0. For Dogecoin for example it's 30. So you could reuse that same library and generate valid Dogecoin transactions just by passing that additional parameter to the pubtoaddr function like so:
pub = pubtoaddr(pub, 30)