Some advice please - I've just bought a chain and sprocket kit (15/45 from Yambits), for my DT250A that I'm restoring. The rear sprocket that came with the kit has an internal diameter of 62mm but the carrier on the wheel has a diameter of 60mm. 

Although it still bolts up OK, shouldn't it be a tight fit on the carrier? The gap is too small to machine a spacer, and I haven't found any where (in the UK) that sells a rear sprocket with an ID of 60mm. Are they avaiable in USA or should I not worry about it?

How well does the retaining circlip sit over it?

Not sure I understand what you mean - the circlip is properly located in the hub and it partly covers the gap and secures the cush drive? 

Might be my old, decrepit memory playing tricks on me!     I had in the back of my mind that the circlip might sit over the sprocket.

I've just ventured out to the garage to check on my own DT360A and RT2, and it looks like it doesn't. Therefore I don't see any obvious issue with that small gap.