You are going to have a big headache fixing it if you haven't mastered gearboxes and gun body repairs.
Learn some Mechboxe tips through videos here -
http://www.mechbox.com/
When I got my first gun a TM MP5K for cheaper than $300 I was happy as hell, it was the cheapest I can find and me thinking TM is a good brand which it is but unfortunately as good as a brand can be the owner and the years of use can seriously wear it down especially when the gun has 3 years + mileage on it
If that UMP is really messed you would end up spending upwards to almost as much as a new Cansoft gun would cost
With really really used guns most likely you have to change
- Hop up rubber
- The whole hop up unit if it is mangled beyond repair
- Wiring (If you see eletric tape all over the place, replace it)
- Motor (Its usually about to die, you can even smell it)
- Gearbox internals