GZ125 chain replacement

[EzyRider]

Right then, been reading a lot of the topics on here for a while, now it's time to try and get the answers to my questions...

I have a Suzuki GZ125 (58 Plate, the K8 I believe) and I think the chain/sprockets need replacing... First, I take it they have to be done as a set?

Second. I believe it is as the 250 and a 15T front as standard? I have read the guide on fitting a 16T front sprocket, would a 125 cope with this? or would it just kill it with it's limeted power?
Looking to get the revs down a bit at the 'higher' speeds...(Though I can only get 70 out of it!)

Will the 250 workshop manual be the same as the 125? I cannot find a workshop manual for the 125 of this age anywhere!

Can anyone point me in the direction of or tell me the detials of the chain and sprockets required for the 125?

How much of an in depth job is a chain replacement? I used to be a mechanic, but never touched bikes...

Sorry for the list of questions! Any advice you can offer is much appreciated!!

Cheers all!