Can you explain a bit (or link to) what the garage MOD does and what the minimal requirements are for you to be able to convert?
Going the fork-way will only temporarily solve your problem since that fork won't run on PHP 7 either. PHP 5.6 will only receive security fixes until the end of this year. After that, it's end of life. And it's already on extended support, so most hosts wil probably drop PHP 5.6 somewhere in 2019. See http://php.net/supported-versions.php
So if you host drops PHP 5 support, you'll have to move on to at least phpBB 3.2. My advice is to look forward and try to work out how you can accomplish that your board will still work with all desired features in 2020. Moving to an uncontrolled fork now will alter your DB and make upgrading properly only harder.
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