Can you undo derestricting of a 50cc moped?

Get your Dad to have a proper look at the leak to check it is what you think it is and if they can fix it. As for re-restricting a de-restricted moped, yes, you can, but you need to know what was done to de-restrict it in order to undo it or re-do it as the case may be. If you do a search on the bike and how to de-restrict it you should be able to get an idea of how to put it back to a 50cc.

It makes such a change to have people wanting to make it go slower lol.

Hope all goes well with it and happy scooting

Helpful to know the make, model, year of 'moped'. Most older mopeds and many newer ones are air cooled engines,some of the enclosed body work scooters with 'moped' license have a water cooled cylinder and radiator at front body work- this seems to be your proposed 'moped?'- and leak looks like it is hose at a crimp/bend or where hose connects to engine or radiator pipe? SO, if you can check manual for the little critter and see how it was restricted, then you can put it back to stock restricted tune- some scooters use a ignition control box restrictor- simple plug/unplug the speed restrictor box or replace with the box for the 30mph speed limit market. Some of the 2 strokes use a exhaust port washer and de restricting means remove it-- so to restrict means simple put it back at front of exhaust pipe where it bolts to cylinder. Some 4 strokes use a similar washer at the intake manifold, sometimes as part of the gasket along with a glued into place set screw at the throttle stop to restrict throttle opening- a strong screwdriver twist breaks the glue hold and then intake gasket is replaced with wide hole gasket for full power capability- SO, again put back restrictive gasket and set the throttle screw stop to legal position. A couple models noted to have a electric solenoid at throttle stop- when speed reaches 30mph speedometer sensor pushes out solenoid plunger to stop throttle movement any more- if you go uphill and sensor indicates less than 30mph then plunger retracts a bit and engine can run a bit faster to get back to legal 30mph limit going uphill when more throttle opening/power needed- solenoid may have been removed or just mounting adjustment backed out or maybe just unplugged from speedometer sensor- usually the plugs are 'sealed' with a shrink fitting to prevent vibration loosening and have a plastic insert catch like some computer keyboard cables might need to get a new wire.

You need to look at the water real close, to make sure it's not the block or head, have your Dad look at it. You can put a bigger carb and change the exhaust muffler to make it run faster. messing with vehicles 49 yrs

Water leak? I would find another moped, and be legal.