Apple Watch 9

Yes, you can. The only thing is that the old charger does not have fast charging as the new one does.

You may use any USB charger, including your Macbook charger, or just a port on you personal computer.

If you decide to buy a new USB charger, from any manufacturer, 20 watts is the ideal capacity, which gives you fast charging.

You can use a higher wattage charger, but the Apple Watch is only going to take what it wants/needs until it is charged, then stop drawing power.