FR125 max engine advice

So I have a Arrow go kart with a FR125 max engine that has been sitting in the garage for over 10 years now and I am looking to get it back up and running but I know nothing about them so I'm after some advice on what you think I should do to get it back up and running in its best possible form. 
I keep seeing everywhere that there is quite a few different types of FR125 max engines and I don't have a clue what kind mine is, so how can I find that out? 

i got a new exhaust for it years ago because the old one had a hole in it but I never fitted in so it's just been sitting in the garage with it, should I upgrade this exhaust again to a more recent one? I want to do all the labour on it myself so if you have any tips on what I could do to rebirth this bad boy it would be much appreciated. 

Cheers! chewy. ?