Rattling noise in first gear. 6 Volt Lady

I have a1975 Lady Dax and runs ok other than when in first gear there is an intermittent rattle noise coming from gearbox i think it rides ok in 2nd and 3rd ? any ideas ???

I'm afraid the first gear sprocket is missing a tooth or two...

Thanks for that,so just a strip down and straight replacement ? anything else i need to do other than just replace ? i will obviously check things well im in there etc look for secondry damage etc.........

You really only need the gear sprocket and a clutch case gasket.
A tool to remove the clutch nut will come in handy as well, or you'll have to take the hammer / screwdriver route + oil filter wrench.

Check for any loose parts and excessive wear (such as the gear drum and shift forks)
make sure you find and remove the missing tooth and clear both the oil and clutch filter.

If you take out the complete gear kit, make sure you keep everything in place. (you can use tie wraps)
Check if the kick start axle is starting to wear down where the kick starter is fixed on. If so, this might be a good time to replace it.

Thank you kind sir

Waynus , ive eventually got round to taken it all apart and you are a genius its just as you said except their are two teeth missing of the lower gear ,ive found it quite straightforward taking it all apart and the hardest bit was splitting the cases , you say "if you take out the complete gear kit, make sure you keep everything in place. (you can use tie wraps)" i have to dismantle them to replace the lower gear etc as you know,whats the pitfalls in doing this and i dont want to assemble it all together and the gears to be wrong/any diagrams or tips on this would be most appreciated.Thanks again Chris.

If you have a diagram (workshop manual) and pay good attention, you should be fine.
You can find them on this site, not sure where (search).
You can keep everything together except of course the part that need replacement.
Check your forks while you're at it.

Well a great christmas break for me ive actually had time to go in the garage and ive rebuilt the engine and all is superb now,runs like a swiss watch once again,what a wonderfull design and simplicity itself this engine really is,just one question for the genius waynus......whilst inspecting various parts etc while stripped i noticed very very very light scoring on one bearing outer race on one side of crankshaft etc almost as if its rotated a little in sittue,is this a problem of any kind ?.Many Thanks for all advice Waynus and a Happy and great new year to everyone.

Some VERY slight scoring is normal. Could be there since the running in 38 years ago.
Just make sure the bearings, play and oil level are well within the tolerances.