Can't comment on cost, but a Perkins
is one of the cheapest engines you will ever come across to rebuild
(not to mention most reliable). It will be far cheaper than repowering with a new engine. You just have to weigh up if you want (need) more HP/weight and better emission control, that is "usually" availble on newer engines.
Repowering is expensive. It is more than just the cost of the engine. Usually a good rule
of thumb, is double the cost of the engine purchase price
by the time it is in and running.