Choosing the proper bearings for replacement
Bearings are of standardised dimensions. In your case 6902 is 15 x 28 x 7 (inner diameter x outer diameter x width) and 6903 is 17 x 30 x 7 (which you probably got from Google).

Thus, dimension-wise, as long as you stick to the original dimensions, you're good to go. In other words, you can safely replace these bearings with others of the same size (6902 and 6903) but different brand and type. For the types, those are used in the industry (full source obtained from https://www.astbearings.com/bearing-closures.html):


ZZS - Removable non-contact metal shield retained in the outer ring with a snap wire.
ZZ - Non-removable, non-contact metal shield retained in the outer ring via crimping, or pressing.
2RS - Molded rubber seal. This type of seal is Buna-N bonded to a steel insert.
2RU - Molded rubber seal, non-contact. This type of seal is Buna-N bonded to a steel insert.
2VS - Molded Viton seal. This type of seal is made of Viton bonded to a steel insert.
TTS - Glass reinforced PTFE seal is retained in the outer ring with a snap wire.


You would want to have 2RU or 2RS since they provide the best protection against moisture at the affordable price. The "non-contact" (2RU) means that the seals are not making contact with the inner ring ("shaft" part) - less friction but less protection against dust and moisture.

Concerning the ABEC scale you can read on https://www.astbearings.com/bearing-tolerances-precision-levels.html - there are apparently more parameters than ABEC rating only that determine the longevity of the bearings. Nevertheless, what I've heard from the rollerskates world - not always the higher ABEC class, the better. The bearings rated higher have slightly more resistance but they benefit from less play. Rollerskate riders recommend going not higher than ABEC 5 as going higher introduces too much resistance in rolling.