Apples, pears & plums

I took over my 25x50ft plot late last year, half of which is occupied by fruit trees and bushes, most of the various apples and a plum tree look healthy, although what appears to be a greengage and a pear tree are bit willowy and less than 6ft tall, with very few fruits. The trees have been there for ages though it was overgrown and neglected. The question is should I treat the runt trees as basket cases and replace or is there any long term remedial action I can take.

Thanks gobs, I also have what appears to be a cobnut tree, but the foliage is the colour of a copper beech and I thought cobnuts were green.