Check the air filter as well. Probably not the cause of the popping etc, but a blocked one might make the bike feel "strangled".

While you have the carb off, make a note of the number stamped on it. Also, if possible, check the jet sizes, the needle number and that these parts have the Mikuni logo stamped on them (a small square in the corner of a larger square). Don't be tempted to buy an aftermarket carb repair kit.

It’s currently running an aftermarket performance filter but I’m looking to try and find the original air box. 

I wouldn’t buy cheap copies, the bike is staying for a long while therefore it only has oem parts. 

Your carb looks like the original from DT125a, worth checking the jetting to see if they changed it from 140 main and 25 pilot to the CT1-B spec of 150 main and 30 pilot.
There is an o-ring (item 5 on the attached Diagram) which is critical to how your bike runs, Yamaha part no.214-14147-00 (standard o-ring size 007) I use the standard 007 in Viton.
The non standard filter probably effects the jetting too.
 

Pete thank you.

Well I stripped the carb, all seemed ok, however the main jet looked To be 150 and the pilot jet 25 therefore a mix. 

I then went on and pulled the points cover and took the pics below, also there was a piece of what looks like plastic in the housing, has anyone got any idea what it is? 

Well I stripped the carb, all seemed ok, however the main jet looked To be 150 and the pilot jet 25 therefore a mix. 

I then went on and pulled the points cover and took the pics below, also there was a piece of what looks like plastic in the housing, has anyone got any idea what it is? 

Looks like the right part for that engine.

Can you pull the flywheel and see if the insides are intact?

@gr8unclel I would have thought that if it had deteriorated inside the flywheel it wouldn’t run at all? 

Looks like a very old packet of dielectric grease, like those little packs you get for automotive light bulbs or for ignition points rubbing blocks, is there any greasy stuff in that packet?

 

It looks like a piece of hard plastic with some old gasket sealer around it.