On E vs F, bumbs, etc.. I haven't had a Kusan in front of me in many decades. Even my reading up on them well was a while ago.
On exacting loco I.D., it is things like trucks, panels, number of fans and where, etc.. I'd just as likely count rivets unless I learn it 'naturally' or hands on (have at it though 
)
On diesels, the draw is more about style for me than anything else so I've never really tried to learn them. I like them all, but I might rather have a model of the motor if you take away the style, because that's all that's left that matters to me.
So I see E or F "carbody's" more as a cool shape, even after the return to traditional frames; ALCO vs EMD; etc. they are all still "carbodies" to me for the shape. The older the better too. I liked the demo/ TA nose rake and smaller consists best. BL2s, Deltics, Little Joe, etc. all fall into that niche or me
Soldering isn't really hard (unless there is some motor skill issues). It just takes some practice and a hot iron or patience to wait for heat transfer. Most issues are simply trying to use plumbing solder, dirty metals (flux cleans it if bad) or not enough heat.
Or rarer, far too much heat (like near red hot, metal changes color, etc.) The fact is, solder moves to the heat. Any issues you had were likely from not enough heat, and the solder wanted to climb back onto the iron because the brass, copper, wire, etc. wasn't quite hot enough. Close, but not enough. When hot enough, solder flattens. Too cold, it forms beads, too hot it forms beads and sweats "dirt" (and maybe changes color of metal) .
Using the F as 2r on 3r track depends on the track type. There is actually a lot to be considered. Loops without turnouts are much easier.
Speaking of easy, converting to 3r can likely be done and removed without a trace. The wheel wipers can stay. You would only change where one sides wire connects inside the train. (where varies by final choices)
They had a switch under other versions for 2r /3r or maybe ac/dc... point is you could also add a switch for 2r/3r if you wanted. Unscrew the roller(s)and unplug (add plug) then throw the switch for 2r...
Track:
You'll need all of the left or right outer rails to be isolated from each other. GarGraves and Ross come this way (wood ties). Tubular track needs isolation fish-paper insterted into one side of all rails, between tie and rail , so that one outer rail is isolated electrically, just like our center rail. This rules out many turnouts. GG or Ross turnouts would be needed too, or every turnout would also likely need modification too, if it can be done to that type.
Plastic roadbed track takes clipping the bands that connect the outer rails together, and doing plastic turnout mods too, if they can be done...??? This is new territory. I've not been inside newer turnouts to know how hard or easy it may be.... my optimism isnt as high on these tracks.
So yes it can be done, but converting to 3r is easier.
A bridge rectifier on the engine will turn AC to DC , but alone on the train means forward only. You will also need an e-unit or reversing board* if reversing is important to you (I'm happy just looping anymore...guilty of being easily amused ) The forward only method will cost you more in labor than the part (under $5... grab an extra 
) B.R.s in the right package style can accept push on crimp connectors if that thought appeals to you.... no solder, just wire nuts and crimping.
If a board is the choice, it must handle the amps of the motor new or old. The amps can be measured pretty easily. Or ask what others with Kusan read off thier motor and assume yours is the same, or has slightly higher draw just to be safe.
Do you have a volt/amp/ohm meter? We can do this stuff step by step if needed. If you feel embarrassed about anything, private contact is ok by me too. My mail is listed, and Im unable to work as well, so I'm pretty available too. Click my name and look around my profile for the address. And fyi, I wouldn't have guessed you had a recent injury if it's any consolation. I can relate somewhat too. I think I had heat stroke about 10 years ago.
An alternate method, mount the B.R. near the transformer and add a toggle to choose AC or DC going to the rails. A second toggle can then change DC directions when needed.
For 3r conversions you would remove one sides wheel wire inside and connect it to the other sides wheel wire. The new roller wire would go where the removed wheel wire used to go. (or to the reversing unit of choice if one is used, etc.)
*Worth noting, a reverse board may have a rectifier built in. The only problem has been the large amp total used by O many older motors has kept these limited to being used on new trains equipped with small can motors until fairly recently.
I'd also consider a search at, or call to DALEE to price a high amp, ac to dc rev. board.
I hope this makes sense. I had to fight this composer for 4 hours to get it this clear ... whole paragraphs vanishing or moving , backspace creates two cursors and erases on 2 lines at a time, etc etc.
