Would be just easier to make sure that your server stays php 5.4 and mysql. Worth the talk with the host what's the prognosis. But since 5.4 is no longer supported sooner or later it will go away.
Jeremy from JMB stopped support around 2012/13. Unofficial JMB does not work anymore as well, I guess due to as Toby mentioned not enough interest in the product.
When I had clitpass.com just before I quit adult I was trying to twinker the LinkX code to php 5.6. But decided to quit due to enormous amount of time needed to do it. Time which I didn't had then.
I started to write my own script compliant with pdo and php 7 on any free time I had. Two years later I'm still halfway there.
Best bet would be to just find new product that is supported and actively updated if you want to start from scratch. ( consider flatfile based products as well , easy to maintain and no mysqli or pdo needed then )
Otherwise beg your host to keep php 5.4 if you want to stick with older scripts. TGPX is still a piece of art that's doing the job perfectly at very low to none cost.