Can I repair manual transmission? Or should I replace the complete gearbox?

These transmissions are not a simple matter, as they are very complex in construction, you should contact a transmission specialist on the matter, it sounds like you have a bad syncro, you will have to price out the difference between having the unit rebuilt or replacing it...I would not recomend taking it apart unless you know how to do so.. If you were gonna replace with a used unit, keep in mind that you would have a 50/50 chance of getting a bad unit or one that may develop other problems, the big benefit of having yours rebuilt is the fact that the internals will be new and you would have some kid of warrenty as well..

REPL WITH R/B UNIT REPAIRS ARE BANDAIDS.