The pressure produced by the old spring could be increased by stretching it to lengthen it a little bit. Not enough to distort it so that it won't fit but just enough to lengthen it.
And conversely, the new spring can be cut and shortened a little bit to weaken its return strength.
It appears that Numrich Arms sells the Marlin cocking handle return spring but maybe you'll be happier with the stronger and more reliable TC spring.
http://www.e-gunparts.com/products_new.asp?CatID=11798