I used the PayPal widget to make a way for students to buy t-shirts for our group. Can I take cash or checks in person as well?

Or do I have to recreate all the work I did in the widget without the widget?

Yes, that's possible. You only have to create a condition in your form that hides the Paypal field so your users will be able to submit the form without any payment.

Here's a guide for that: http://www.jotform.com/help/120-How-to-set-up-paypal-along-with-other-payment-options

If you need further assistance, let us know.