i have heard many good things about the 4.6 engine being a strong runner.
As far as a "collectors item" as much as i love the MGT trucks, i dont really know if it will change its value, or become a collectors item, unless it were a NIB, or highly upgraded. in the near future( 5-10 years) it might be something more. even now BNIB MGTs are going for right about sticker price of new back when released, which could be argued for inflation, be less.
kinda was the era of upgrade everything, and many manufacturers for said parts. on the flipside to that though, being all original could eventually increase the value over a slightly modified, but parts from companies out of business or no longer make parts, are still going to probably outweigh the cost of being original. (HCR, New Era, Novaracing, Hardcoreracing, UE, FLM, etc)
that said, on my "MGT" (the original .21 version,) i rebuilt the engine, everything except for crank, and and carb itself, replaced o-rings though. i did purchase the FT cooling head for it but have held onto the original.
i got really lucky on finding parts on sale, AE was doing a clearance sale on parts about a year back where my engine order crankcase, backplate, piston rod, wrist pin, FT cooling head, etc, was 40$, then found a Piston/Sleeve for another 40$.
overall its up to you what you want to do with it. they are some fun as hell trucks, i have 2 at the moment, the one im going for a more lightweight "racer type" build, the other is heavy basher build.
personally i would keep it, and get like a big red for 150$, or a nova for 220$ and run it. then as you find replacement parts for the engine, can rebuild if you want to have as a backup.