Most economical is a fresh 2.3l, you could Cam it or even turbo it for better performance.
Turbos are nice since you get to keep the 4cyl MPG but can burn the extra gas when you want the extra power, a larger engine burns the extra gas whether you want the power at that time or not, lol.
After that would be to buy a pre-'95 4.0l ranger with the trans you want, i.e. manual or auto, and 2WD or 4x4 to match yours.
This will be the donor vehicle, there are vehicles that have been rolled or in accidents that have been "totaled" by insurance, these can often be picked up at a reasonable price, what is reasonable is left up to you.
Then you start swapping parts.
2.9l wouldn't be worth it.
1995 is when the Rangers got the newer EEC-V computers which adds more cost to a swap, they also got electronic speedo.
1994 and earlier used the EEC-IV computers and mechanical speedo cable, like your '92
4.0l OHV engine was offered in 1990 to 2000 Rangers
V8 swap will be a little more costly.
But if you can find a V8 explorer for a donor at a reasonable price it can be done.
Has to be a '96/'97, in '98 explorer got the PATS anti-theft system which adds an extra layer of cost to integrate or delete.
It will also be an automatic trans, not alot of 5-speeds were made.
And EEC-V computer
Lots of good info here on all the swaps:
http://www.therangerstation.com/tech_library/index-conversions.shtml
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