If you already have the RSAPublicKey instance it is easy. There you have to methods for getting the modulus and the exponent. Both get-methods copy the requested value into a byte array you can return in an result APDU.
Note that for a common JavaCard without extended length APDUs a 2048 bit RSA key is to large to return it in one response APDU. Depending on the available bytes in the response APDU you may even have to split modulus and exponent into 2 or more parts for returning them to the host.