I agree with the above, about adding the missing rotor. Two ways to do that: simply get a secondhand stock wheel, which you may be able to find with both rotors plus a tyre; or look for a suitable custom wheel, however that can be very expensive!

The PO appears to know nothing about what he was doing with the front brake, so it is 'safe' to assume there may be other unsafe traps around your bike. Be very thorough in checking that your bike is trustworthy.

 

Ask PO if they have original wheel and rotor.

 

can you fix it with a hammer