Hi I've bought a cheap 1976 Yamaha yz 125 it was cheap because it has a DT 175 engine. I don't think the guy who did it had to modify the engine mounts or anything just by the look of it the yz airbox is gone and a make shift air filter has been made. Also it looks like the yz exhaust has been kept but cut buy where it goes to the head and a DT bit of exhaust was welded on the fit the DT engine. It's a mess, and I'm sure I'm losing power here as it's pretty slow I mean it doesn't even power wheelie when slipping the clutch in 2nd. 
Has anyone got any tips on what to do to make this better? 
 

So many possibilities. I always go fo exhaust restriction. But no use guessing without us hearing how it sounds on a video. Does it rev out cleanly in neutral or sound limited & flat. HS it got good compression to feel on the kick start. Welcome if you're new by the way. Videos need to come via YouTube or similar. I guess that's obvious but I meant photos need to come via something like Imgur to show on this site.

Greetings from Tennessee  

Welcome from Michigan. My situation was the reverse of yours. I bought a DT175E and shared photos here on the forum. It was MarkT that informed me that I had a Frankenstein bike. Mine has an MX125 engine that appears to fit the frame well. The exhaust appears to have come from a donor of different descendants…. I haven’t done anything but roll it into the corner of my barn.

imgur.com/a/jG74KkD

Engine mounts are the same for the left hand chain 175 (and left hand chain "kick start only" 125 models) after the 1974 redesign through at least 1981.  Other things like pipe and intake might not match up, might need to trim fins, possible downtube interference, etc.  But in most cases it's an easy swap.  Even an RT180 or Blaster engine will fit the mounts.

(Note that the USA 74-76 DT125 all had electric start...  the 125 e-start models kept the old 69-73 AT/CT engine mounts)

Really hard to give too much advice without a ton more info.  A good running DT175 with the extra 50cc's has close to the power of a 76 YZ125 but it's a lot more "mellow". 

And keep in mind a modern stock YZ85 has way more power than a 1976 YZ125...  these bikes were slow and had a lot less power compared to modern bikes. 

YZ pipe is not going to work optimally with DT porting...  YZ pipes are tuned for 10,000+ RPM peak power and the DT175 porting peaks several thousand RPM below that.  Quite a mismatch. So you probably have less power than a stock DT175.

Other unknowns are carb, engine condition, and even which DT175 engine you have. 

What I would do is port the DT175 for more power and refresh to make sure it's in good shape...  Someone that knows porting these old engines will be able to get the DT porting to work with the YZ pipe pretty easily. 

Or if you're looking for a whole new engine, my choice would be a 77-79 IT175 but there's a lot of options. 

You said it was cheap...  and another option is just make sure the engine isn't too worn out...  timing and jetting are good, etc.  Ride it and ENJOY. 

Mark, what frames will the IT engine drop right into? Not the CT/AT but 74 up?

74-81 DT175 and kick start only DT/YZ/MX/IT 125 frames with left side chain. 

For example, 78-81 DT125 was kick start only.

And we're talking "mounts".  Could be challenges with shifter, kick start, air filter, pipe, chain alignment, and possibly even head/cylinder hitting frame.  The couple swaps I've done have not been hard but there could be issues with some combos.

77-79 IT175 has a 520 chain vs 428 for example...  I put a DT175 engine in an IT175 for a friend and the chain mismatch and alignment was easily solved with Yamaha parts.