How to convert RSA public keys to one key to use for text hashing?
An RSA public key is made of 2 parts, the modulus n and the exponent e.  They are both required, and neither alone can be considered "the key".  Often e will be one of just a few different values, but this is mostly convention and shouldn't be assumed.

RSA does work on integers; if your data is of another form, you have to specify how to convert it.  For instance, if you have text you might encode it to a byte array with UTF-8, then take blocks of bytes such that the number if bits is less than the length of n, and then treat that byte array as one big integer (either little-endian or big-endian).

But you probably don't want to be doing this math yourself, and there are a lot of pitfalls (padding, etc) to trap the unwary.  You should be using a library that someone has already implemented.