The Marksman Repeater is made so poorly that it is not worth repairing. In excellent condition it is worth $15. Broken it is worth nothing. A new model 1010 cost under $20
If you want to pay more than it is worth, look up "Air gun Repair" on the net and find someone near you to rebuild it.
No-one seem to have a diagram on the repeater but I did find this on the web.
See the attached link below
Scroll down to the bottom of the page and you will find photos of how this guy put his back togather.