If a boat has no data plate how can you determine motor size?

Is there a formula with the hull size / weight?

If I stand on the transom and the boat is still stable does that mean I can put a 200lbs motor on there?  

This might sound stupid to some, but it you have been reading my posts, you know I am brand new to boating and I know next to nothing. So I could use all the help I can get.   Oh, and type slow so I can understand

Here in PA all boats have to have a Max Load/HP plate.  I had an older  boat without the plate  I wanted to register and put a motor on.  There is a form on the PA Fish and Boat Commissioner Web site i had to download, fill out with measurements and send in.  They sent back a plate for a fee.

So there must be a formula they use.  Maybe you could check with your own state's boating regulatory agency.

Coast Guard Horsepower Rating System

The formula for determining maximum allowable horsepower for outboard powered boats under 20 feet in length and with at least a 20" transom height is: (Lft x Wft x 2) - 90.  It is permitted to round UP to the nearest 5 horsepower. The manufacturer may also rate his hull at LESS than the maximum allowable. Tiller or stick steering applications have lower ratings as well. All hulls produced after mid 1978 must also meet the "positive, upright floatation (paraphrase: when filled with water at rated load, the boat will not go down by the stern, nor will it flip and go keel up)" standard. For the full text (heavy reading) of both the Hp and the floatation regs, you may visit the USCG website and view the following: Code of Federal Regulations, 33CFR183.

Horsepower examples

17' x 72": (17 x 6 x 2) -90 = 114 Hp

18'10" x 86": (18.83 x 7.17 x 2) -90 = 180 Hp