The engine that my van came with(the engine i just scrapped)was in a bad state, it was completly gutless, the pistons had been hitting the valves, two piston heads were very badly damaged no doubt the valves would have been completly knackered too i didn't even check! The head was also damaged beyond repair, so i scrapped the engine and bought the donor 1V.
I'm definatley no expert on any level but i'm learning all the time on here, the 1V is the first engine i've worked on. The loss of power and smoke could be retarded pump timing? Maybe the timing has slipped.
Have you had any over heat symptoms? Is there any oil visible in the coolant?
I reckon most of the T25 engines have clocked up a fair few miles maybe now is a good time to do a overhaul and see whats going on inside before commiting to buying a donor engine. If your gonna take your current engine out to make room for another you may as well crack it open and see whats what.
