There probably will not be any information forthcoming. You're probably within horsepower limitations on that boat, or maybe 5 hp too high, but not to worry.
White's Fiberglass apparently still exists, in Groveland, Florida.
22036 County Road 33
Groveland, FL 34736
(352) 429-2700
Why not call them on the phone? Have your boat's ID number handy. They can probably tell you what you want to know.
There still should be a capacity plate on the boat somewhere, probably near the console. Lots of times, they get covered up by something, or painted over.
Apparently, they didn't make a ton of Johnsen boats, so there's not a lot of info on the web.
Call 'em!
Update: They appear to be out of business, according to the Coast Guard site, but the business may still exist:
http://www.uscgboating.org/recalls/mic_detail.aspx?id=WFG
Additional update: The molds from WFG were acquired by another maker. Here's that maker's info:
http://www.uscgboating.org/recalls/mic_detail.aspx?id=OLE