Dangers of overloading a 2.5A rated plug
The risk is if the plug (or its contacts) heat up too much, they can cause a fire in the wall of your house.

I'd go buy a correctly rated plug rather than take that risk.

Also, if the wiring inside the appliance is "scary", you'd be wise to use a grounded plug and ground all user-accessible metal parts, to reduce the risk of shock.

Of course, to really reduce the risk of shock and fire, you'd be better off cutting the cord off this thing and displaying it on a shelf, while buying a new kettle with proper safety marks to make tea with.