I need someone to explain to me, as if i was a numpty… what size chain i need 
I have a 32mm OTK TDX chassis, with a rotax evo rocking a 75 rooth rear and 12 tooth front.
I know there is pitch and stuff but is there a simple “get a 104 chain” answer?
Get a good 219 type chain and shake and bake my son !! Laugh 
I am using a Japanese brand
That sounds like a 219 pitch chain to me. If you don’t know how long of a chain you need, just get a 110-114 link chain and a chain breaker, then shorten to suit your motor spacing and gear sizes. If you are just running for fun, throw a quality O-ring chain on it and it will last a very long time. If you want to squeeze that last 0.1 to 0.05 sec out of it, put something like a DID HTZ non-oring chain on it and you should be good for lots of sessions.
The engine can move back and forth to accommodate several lengths of chain, so it will depend on what mount you use etc. on how long of a chain you need. 104-108 is fairly good for most gearing in my experience. If you run a standard chain you can break it and add or remove links as Chuck noted, so length is less important. If you run o-ring chains then you can’t break them and you will need to purchase the correct length.
I never ran o-ring on Rotax back in the day and didn’t ever have issues but I’m not sure if that has changed with the Evo.
Also running similar gearing on a Rotax. I have a 104 link as my go to and a spare 106 link for running a bigger rear or if I pop a chain. Your result’s may very depending on if you have extra seat struts you need to miss with the electronic bracket coming off the back off the engine or if your chain tensioner is too long or too short.
Buy an o-ring chain if you can afford it. Example link below.
Bless everyone’s bones. I appreciate it. I’ll get myself a nice 104 o-ring.
Thank you all.