Can't find any replacement original chainrings (SG A-52-42-30) . What can I use instead?
Yes, you don't have to use actual Shimano chainrings, you can use 'third party' brands.

There are a few manufacturers that produce replacement chainrings in a wide range of BCD measurements and tooth counts, so you be able to get exact matches.

If you do decide to change gearing, derailleur position and chain length will need to be changed, but you should probably be replacing the chain at the same time as as rings anyway, and also checking at the cassette for wear and replacing that if necessary.