I'm not a lawyer nor did I stay at a Holiday Inn Express last night, but two extremes to consider. Hawk Model vs Aurora. The founder of Aurora, Abe Shikes, visited a hobby shop one day to get some bird seed and while he was there saw some plastic kits by Hawk for something like $5 a piece in 1950's dollars (like a million dollars today ). Being in the plastics business himself he couldn't figure out why they cost so much and why people were buying them at that overly inflated price. After some time of studying of measuring the volume of plastic and other costs, he decided to get into the plastic kit business and issued two kits for 98 cents a piece. Needless to say they were a hit. The problem was that they were a direct copy of two of Hawk's kits. The legal wrangling went on and on, but in the end it came down to the instruction sheet. Aurora copied things so precisely that a series of dots and dashes ( .... .- .-- -.-) were included. When a lawyer for Hawk asked about them, no one had an answer. When it was pointed out that they spelled H-A-W-K in Morse code, the case was over. Now that was an active issue kit against an active issue kit.
The other side of the coin would be Polar Lights. The early issue kits were not from molds that were leased from Monogram, they were recreated from the pieces of original Aurora kits. As to who owns the intellectual property - at least the name Aurora is owned by some company, the molds were purchased by Monogram - who knows?
Another point that just came to mind is in regard to not just the model, but the form itself. A lot of the subjects (cars and science fiction subjects come to mind) are copyrighted by another company and would likely require a separate licensing agreement. For instance if you were going to issue a model of a '68 Ford, think again. Model companies have a licensing agreement with Ford for each model that they issue. Even military aircraft are becoming this way. I don't know Boeing's stance, but I've definitely seen Lockheed aircraft models with a blurb that says that the product was licensed.
I would agree with a previous poster that if you were going to take a model and copy it for yourself and/or a friend is probably not going to attract much attention.
My 2 cents.