Hi everyone. I am a complete newbie. I have a heavily modified bike that I think is a 1973 Yamaha DT 125B with a 1978 engine. When I bought the bike 15 years ago, it looked like the first picture. A previous owner painted over the original red color to make it yellow, then painted the gas tank to say 175 instead of 125. That leads me to believe the engine was modified (bored out?) to 175, but I have no way to tell. Based on the motor serial number, it is a 125.It was also stripped of most of the street legal things such as mirrors, lights etc, then it was made to run without a battery. The wiring is a mess of cut wires and tangles.I have stripped it down to the frame and want to start rebuilding it. I am out of my league on pretty much everything I am doing, and I am looking for help and/or even a mentor. Online assistance via this forum will help, and a local (Vancouver) mentor would be best.My first question is do I yank out the remaining wires and start from scratch, or do I try to rerun the existing wires?Any and all help is appreciated.  

Sorry, at this time can't upload photos  

That is a common upgrade since the top end goes right on with no modifications. You can look at the bottom of the cylinder on the shifter side of the engine and it should have a casting of the cc's on it.

 from near Salisbury, UK.

I can't do it myself, but I'm told that you can upload pictures to a photo sharing site and then post links here to that.

As Jim said, putting a 175 top end onto a 125 was common practice back in the day.

The general wiring isn't as complicated as it might first seem, as it is all colour coded, even between different models. However, I don't know enough about your model(s) to comment on what to do regarding the stator/flywheel/lighting etc area.

Greetings from Tennessee. 
  You can go to the tech library and look up your serial number to see what you have could be a At1 if you’re gas tank was originally red is your wiring all cut up? 1973 would be a At3

I replied earlier and my post didn't save. 

1978 125 (or 175) engine has different mounts (lower rear mount is lower) from a 1973 or a DT125b (USA model)..  (plus there is no such model as a 1973 DT125b)

First step is clearly identify what you have...  Check VIN of frame and engine and/or post numbers here. 

Since you're not in the USA, your country may have received the twin shock "6v magneto" 125...  If so and if you have a 1978 engine that would make wiring easy. 

Thanks very much for all the replies guys .I really appreciate them. Sorry for the late responses to your replies but I didn't get notifications that guys responded. I have this time checked the notification box below.

Here are the serial numbers:

Motor 2A6-020939 Frame 444-117190

I will try to figure out how to upload photos so you guys can see the bike. Interesting about the 175 conversions. I will look for the markings

Link to photo before: photos.app.goo.gl/aHMGz1go1KPUY7Jk7
Link to photo now: photos.app.goo.gl/gkSZHFZDtane9jxv5

Please let me know if the link works.

the cylinder seems to only have "25" as a marking.  Here is the link     photos.app.goo.gl/2nomxZvuPygzfL7A6

Frame is a 1975 DT125b. 

Does the lower rear engine mount on frame line up with engine?  Or has someone made a new lower rear mount?

Your last picture is of piston, not cylinder...  should have displacement cast into cylinder on lower left side edge. 

OK. Thanks for that info on the year of the frame.  Right about the difference between the piston and cylinder. I did know that, but got them mixed up.

I will look on the cylinder for the marking. It would appear that the mounting points on the frame  line up with the engine itself.