WTB 1300 engine for a GT Junior

[Thevak]

Helping out my son with a recently seized a motor. Yet to determine what caused it exactly but oddly cylinder 3 has a spark plug that is difficult to remove (any suggestion).

I am keen to help him by sourcing a replacement motor that is currently a goer but no longer needed by past or current owners who have upgraded (he wants to maintain integrity and is not wanting a 2 litre ). Hoping to get it back on the road and track as soon as possible (finance and time permitting)