Originally Posted by StephenH

"left abandoned" sounds an awful lot like "I stole this bike".
If it was out in a corn field or something, you might want to check with the police and see if it was ever reported stolen prior to that.
Handlebars/saddle- looks like a setup for a very tall person- you can possibly buy a shorter stem to drop the handlebars down a good bit. You can adjust seat fore/aft and there are some set-back seatposts made, I believe.
"Second wheel up front" sounds like a bad idea. One thing to check is if there is excessive slack in the steering mechanism, which would make it pretty wonky. You can get some reverse-tricycles (Worksman makes them, others as well) but they are heavier, geared more poorly than this, etc. And unlike you might imagine, two wheels up front does not help stability unless you are going very slow or stopped; otherwise, it hurts, as you can't lean the bike into turns.
Gearing for climbing with an adult up front is going to be a challenge. My suggestion is, run it by a good bike shop and ask "What can be done?"
Simplest solution is keep the existing gearing and use smaller chain ring or larger rear sprocket for lower gearing.
With a normal bike on a steep hill, you'll get down very near the point where you can't balance anymore. With about 2.5 times the weight, if you do get incredibly low gearing, you may fall over. If your passenger is not incapacitated, have them jump out and push would be the way to go.
Assuming that is a factory-made bakfiets, I would highly recommend against cutting it up and splicing more tube in, etc. If it doesn't fit, sell it and buy or build something that does instead of ruining a good bike.