Once you research what the capacities should be, there are a couple of custom capacity plate manufacturers you can contact to get the plate made (I'm sure there are more).
Here's some links:
https://www.garzonstudio.com/boat/capacity-decal.html
https://www.outboarddecals.com/index.php?cPath=78_79
Also, if you're using the boat before you find out the capacities, here's a rule-of-thumb to use to figure out the capacity. This is from a PA Boater's Safety Course:
Calculating Your Boat’s Capacity
On boats less than 20 feet in length with no capacity plate, use the following rule of thumb to calculate the number of persons (weighing 150 lbs. each, on average) the vessel can carry safely in good weather conditions.
Number of people = vessel length (ft.) x vessel width (ft.) ÷ 15
For example, for a vessel 18 feet long by 6 feet wide, the number of persons is 18 times 6 (or 108) divided by 15, which equals seven 150-lb. persons (or a total person weight of 7 x 150, or 1050 lbs.).