Advice from a Cheapskate
With a little care, it is possible to remove the innards of most locks in a way that at first glance they appear to be whole. They may then be operated with any similarly shape/size key.
If you set out with that objective, it is most likely that dirt will be the cause of the malfunction. The answer will be dismantle, clean and lubricate. Locks require a dry lubricant such as PTFE powder. Oil dries and becomes sticky. Add environmental dust and mechanisms clog, ceasing to work.
Often, it is possible to remove anti-tamper bolts with gentle tangential tapping from a centre punch. However, access is the issue. Have some machine screws to hand. DirtFlyer says 8M and that sounds about right. In these circumstances, do not bother with the more expensive anti-tamper screws. I would go for Allen cap head machine screws.