Is it a good Idea to replace a worn out engine.

You already have a used engine so why buy another one like it? What do you mean by "worn out"? My '88 Grand Caravan is approaching 400,000 miles and is still running fine. I regularly drag around a tandem axle enclosed trailer with it that's bigger than the van. 300,000 miles on yours by itself is not necessarily a good indication that it's worn out.

If you're having a serious problem, my vote is for a rebuild. You already own most of the parts needed to build an engine so why buy some more? The block, crankshaft, connecting rods, and cylinder heads get reused. Just the wear items like bearings, piston rings, oil pump, and possibly the pistons will be new along with a new gasket set. Everything will be carefully measured and hand-assembled. A lot more care will go into assembling it than did originally on the factory assembly line.

We always told our students who planned on building an engine to expect to pay $2000.00 to $2500.00 for parts and machine shop work. That includes a set of core plugs, new antifreeze, new belt(s), exhaust gaskets, oil, and things like that. Remember they were doing the assembling and installation themselves. You will likely be paying someone to do that, but you'd be paying for removal and installation regardless of which engine you use.

I personally am happy to read you're keeping an older vehicle on the road. Newer ones have way too many unnecessary unreliable computers to do things we never needed computers for before. You'll have much fewer problems with your vehicle. Just look through the forums here to see all the unusual problems people having.

I agree, mostly. Today the crate motors are pretty cheap too, if time and hassle is a consideration.