Upgrade interspire shopping cart 6.1.1 code to work on PHP 7.2 and mysqli

We have an old version of interspire cart (obsolete about 5 years ago) which is running on php 5.5 and mysql 5.6. I have setup a new cpanel server and copied all the data over. Now i need a coder to go through the code and update it so it works with latest php and mysql. The site has a lot of custom code and has been developed further after the interspire became obsolete. There will be places where you need to update the syntax and code so its important to know the differences between the different versions of PHP and Mysql and server administration. It will also be an advantage to know Interspire Cart. Theres 2 servers, one is LIVE active running - php 5.6.3, mysql 5.5, the has all the identical data but running php 7.2 and mysql 5.7