Im replacing the 630 chain and sprockets to 530

I have a 1980 KZ750 LTD with 630 chain and sprockets. I want to replace with 530. will a 530 18t front sprocket fit my bike without modifying the engine.

If you are putting on new chain and sprockets then for sure go with 530.

In figuring out what you need divide everything by 5 and multiply by 6.

Count the teeth on your sprockets or look for the tooth # . Say your 630 set up is 13T front and 33 T rear then dividing by 5 and multiplying by 6 and rounding you will need a 530 front sprocket with 16 teeth and a rear with 40T.

You can count current chain links and do the same same (assuming you have stock swingarm etc) . Make sure you buy good quality chain like DID RK or EK and good sprockets like JT brand. I did this a couple of years ago. Got and EK chain and think I only had to adjust it once
Good luck

stock is 13/32. 16 in the front and 39 rear will be the same ratio as stock.