WordPress Booking Plugin Changes

We have recently purchased a plugin for WordPress, which is used for clients to make bookings online, and are looking at adapting an existing bookings system for use in an occasional childcare service. As a result, we have chosen the WP-OBS plug-in for WordPress. The booking plugin meets many of our requirements, although there are a few changes we wish to have made to both the front and back end. A majority of the work required is cosmetic, although there is some database work that need to be done (adding additional entities, linking them to existing ones) and amending a bookings form (adding additional fields). After contacting the author of the plugin, they are not willing to make the changes, although have advised we are free to alter it to meet our requirements (as they often perform customisation for clients, but have stated their workload is too full to assist us at present). We would appreciate it if you took the time to view the attached documentation before placing a bid, as this is the second time we have listed this.