If it's your equipment, you can do as you wish. If you are restoring for someone else in my opinion you shouldn't modify the equipment. If you do so, you are taking on responsibility for the subsequent safety of it. If you state that it complies with the latest safety accreditation, you may be asked to prove this in the event of an incident. To safeguard yourself you will therefore need to get it independently tested - at great cost.
When repairing for 3rd parties, in my opinion, you should repair and restore the equipment to the original manufacturers design. You should then explain to the person you are restoring the equipment for what this means in terms of safety. Documenting this and asking them to counter sign would also be a good idea. i.e in AC/DC sets the chassis is connected to the supply and if a non polarised plug is fitted it could be at Line potential. Also, the inadvisability of leaving some equipment switched on unattended.
I would also look into insurance if you are thinking of repairing equipment commercially. Sorry if this sounds legalistic and harsh, but an incident could wipe you out financially - that's the world we now live in
