The PayPal IPN in v1.3.x doesn't use CURL for anything, so that's a red herring.
You need to make sure your new server can receive communication from PayPal, as well as initiate it: http://www.zen-cart.com/wiki/index.php/PayPal
As to CURL in the zc_install process, it uses the same code as curltest.php does, but without the ?paypal=1 on it.
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