I have crossed the Beartrap Lake to Beartrap river portage a few times. It is not terribly difficult but the only maintenance is done by paddlers so expect some blowdown to navigate.
There is a little flow to the river and a few spots where rapids, rocks make you get out and portage. The portages along the river are generally short, sometimes a bit of a challenge but not anything you can't handle.
The area along the river burned a few years ago but it has been regrowing-you will just be able to tell it had burned.
There is a decent spot to camp on Sunday on the north shore near the portage headed over to Sterling.
If you go to this trip report you will get some information from a past trip along with some photos
Stuart to Fourtown trip report