What is wrong with my honda cbr 125?

I recently bought a Honda CBR 125 and for an unknown reason it cuts out when i'm idling or slowing down. It has been doing this since i got it but in order to prevent it doing this i have to rev it. I have been told that it could be my ECU but can also be my exhaust. When i bought the bike it had a yoshimura exhaust attached and there's no compression. I thought id ask on here as i do not want to buy the wrong part and waste money on something that will not solve my problem. If anyone can identify the problem with the very little information i have given, it would be very appreciated

Based upon the little you have told us it sounds as though it's fuel starved as a result of the after market exhaust.

As mentioned, reading the spark plug will offer an indication of a lean condition.

If lean, the options are to replace the exhaust with a stock one or adjust the processor to match the pipe.

If you can pull the spark plug, inspect the color of the inside porcelain tip and repost your question with more information, maybe someone can help.
Just a guess is that aftermarket exhaust is leaning out your air/fuel mixture and the jetting needs to be adjusted.

If there was no compression the air and fuel mixture would not be compressed and thus the engine would not run at all.
Your question shows that you have a limited knowledge of engines and bikes and you are going to need hands on help with this from someone who knows what to loook at.
No one can identify the problem fron that description. Some of the many things it could be are -
Carburettor set up, particularly idle speed (do not mess with the carb unless you know PRECISELY what you are doing
Spark plug, plug cap, plug lead, coil problem
Carburettor not set right for replacement exhaust
No reason to suspect an ECU problem