Edit PHP OsTicket ticketing system to make one ticket per email, instead of opening tickets by checking subject

Hi, I am using Osticket 1.15 and currently the system automatically fetch emails from the server, and check the header if the ticket number exists on the OsTicket, it just re-opens the ticket, if it doesn't exist it opens a whole new ticket, what we need is to disable the check by subject and make the system to check by user email, if the user email has a ticket on system before, just re-open the ticket.

My budget is 20$ for this and it needs to be done today.