Only for mempool? Use this patch and compile Bitcoin Core from source code, https://gist.github.com/luke-jr/4c022839584020444915c84bdd825831. You just need to follow compilation guide from https://github.com/bitcoin/bitcoin/tree/master/doc#building. But if you don't want to store any Ordinal data on blocks, you should run Bitcoin Core 0.12.1 or other client which doesn't support SegWit which means your node doesn't store any witness data.