.as far as I know, the reputation in many forums is a separate thing, the exhibiting of which should be supported by something. And if it will depend on reactions, then this is not quite a reputation
It's definitely a separate thing but how you achieve it should factor in your current stature IMHO.
Adding a new workflow to give reputation to users will not get much traction on existing forums especially with an older audience. People don't like to click new things as you know if they're used to hitting like.
You can even simplify it to something like:
Likes-to-Posts Ratio × Total Likes = Rep Points
Now if I want to go out of my way and give someone an actual rep point I can and it'll be worth a lot more than a Like.
You can then allow admins to change those ratios. This would instantly give your seasoned members reps rather than dealing with complaints like I've been around for 50 years and I have a reputation of 0 suddenly.
These are just examples btw, you can use whatever ratios you want.
Simple contributors to consider:
User Defined x Likes
User Defined x No of Posts
User Defined x Threads
User Defined x Media
etc etc
I'm concerned that saying hey members you all have a reputation now and it's at 0 is going to be a negative experience for long timers on my forum. They will likely ignore it or get upset. Starting with an existing value that people are used to i.e. likes is a better spring board.