Change the Captcha of a Magento Guestbook Plugin

I want you to change the captcha of a Magento guestbook plugin. It doesn't matter which captcha we will use, it only has to be spam-proofed. Just give me a recommendation. The used guestbook plugin: [login to view URL] The used captcha: re-captcha The used Magento Version is 1.6.0.0 You will find the plugin source files in the attachment, so that you can take a look at the core files to better estimate how much work it would be.