Before you spend $3k on engine replacement you might want to consider some other things. 90k miles should be nothing on an engine, if you've kept it maintained. If you bought the van new or not, at 90k miles there will be some maintenance issues that will come up. Parts do fail and you're at the point where things like water pump, a/c or transmission

, could require replacement. You really don't know at this point unless you have some current symptoms.
The engine and transmission are the main cost concerns when a vehicle gets near or above 100k miles. Either one will usually exhibit some indications that a problem requires attention before a complete failure occurs.
I'm all in favor of preventive maintenance except for engine or transmission replacement. If neither one is giving you problems, don't worry about it. If you're really concerned, I'd spend some time shopping around for a replacement vehicle instead. For what you would have to spend to replace the engine, that same money would make a good down payment on a $6k-$7k replacement van with fewer miles.