Adam,
pardon me but I am with IO540 on this one. I've seen too many airplanes sitting in a corner without an engine because the owners could not afford to bring up the money for the overhaul, either because it is unscheduled, that is way before the 2000 hour TBO or because they flew happily up to 2500 hours on condition and then got the shock of their lifetimes when they heard the price of the overhaul once it was no longer avoidable.
If you have enough on the side to pay for a new engine any time, you are ok to fly without an engine fund, otherwise, I'd strongly recommend it. Way I did it is to get the airplane it's own bank account where all costs are paid in and out of. The about £ for the engine fund go into a separate subsection, so they don't get used for anything else. I have a brand newly overhauled engine in mine now, but I don't want ever to get in the situation where my plane sits on the ground because some £20k bill comes up which I don't have the means to afford.... there are enough "project" airplanes out there on avbuyer and planecheck for that very reason.