can i just rip it open and start changing any oscillator i can find ?
Only is you really want to ruin it, so NO.
As dzseki has hinted the first thing to check is the PSU is within spec.
Often there's enough clues on the PSU PCB or other PCB's to indicate the correct voltages but you are "flying blind" without a service manual.
All the low voltages, say 30 and less should be within +- 0.1 V of spec with the exception of any that aren't regulated, they can be 2-5 V from spec.
All regulated voltages should have low ripple too, this can be
checked with a DMM set to AC volts that will show the ripple riding on the DC rail.
That the amplitude and timebase are below spec suggests DC supply rail/s might be dragged low by a failing component, possibly a dried out electrolytic or a shorted tantalum cap.
Careful study of the PCB's might enable you to identify parts of the circuit to focus on but it's difficult without schematics and/or significant experience.
But regardless of having the service manual or not the PSU is the first thing to check and it must be correct as the voltages supplied are the reference/s for the scope inherent accuracy.
This doc from the sticky in the Repair board is of compulsory reading when attempting to fix CRO's.
http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf