Convert an RSA PKCS1 private key string to a Java PrivateKey object

I have a RSA private key stored as a String that I need to convert into a PrivateKey object for use with an API. I can find examples of people converting from a private-key file to a string but not the other way around.

I managed to convert it to a PrivateKey object but it was in PKCS8, when I need it to be PKCS1, and I know Java doesn't have PKCS1EncodedKeySpec

byte[] key64 = Base64.decodeBase64(privateKeyString.getBytes());
KeyFactory keyFactory = KeyFactory.getInstance("RSA");
KeySpec privateKeySpec = new PKCS8EncodedKeySpec(privateKeyBytes);
PrivateKey privateKey = keyFactory.generatePrivate(privateKeySpec);