Although the key is too small for practical use, you may still use it for educational purposes. Note that the key is so small that you cannot even use PKCS#1 padding modes, only "raw" RSA encryption (i.e. only the modular exponentiation part of RSA).
The following works perfectly well for the Bouncy Castle provider (where the key is a 64 bit key):
final Provider bc = new BouncyCastleProvider();
// generating the key from modulus & private exponent
KeyFactory rsaFactory = KeyFactory.getInstance("RSA", bc);
RSAPrivateKeySpec spec = new RSAPrivateKeySpec(key.getModulus(), key.getPrivateExponent());
RSAPrivateKey testKey = (RSAPrivateKey) rsaFactory.generatePrivate(spec);
// using it in a raw cipher
Cipher c= Cipher.getInstance("RSA/ECB/NoPadding", bc);
c.init(Cipher.DECRYPT_MODE, testKey);
c.doFinal(new byte[] {(byte) 0x7F, (byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0xFF, (byte) 0xFF, });