Bro i am no expert but can add a little. Pls do consider the following points.
1. In a stock potohar/ sj 410, if you fix a 1300 engine and gear, the gear will come on the same foundations but the engine would require alteration.
2. Keeping the engine cool, you'll have to change the radiator also.
3. If your problem is high rpm on long drives, you can change the tire size also.
4. If you don't want to do that, you can change the diffs also. That's going to solve your problem.
5. What diffs are you currently running. I think it should be 9:37.
6. Just by changing engine and gear and keeping the stock 9:37 diff, your problem will remain the same as the 1300 with a 5 fwd gear has come either with 11:41 or 11"43 diffs.
7. A stock 1000cc engine has no power steering and survo brake option.
8. The 1000cc produces 43 bhp's. You can consider the jimny 660cc intercooler turbo, producing 53 bhp's almost close to the 1300cc engine. And the 660cc has both the power steering and suvo brake options in it.
9. The price of a kabli 1300 cc engine is 1,25,000 and gear is 25000, where as the price of 660cc epi intercooler turbo engine is 30,000 and gear is 5000 which is peanuts.
10. The average of 1300cc is 8/9 km/ ltr where as the a 660cc does 15 to 16 km/ ltr.
11. The 660 cc both engine and gear will exactly fit on the existing foundations.
I hope all this gets you to some conclusion. If not you can always get yourself clear and ask in detail.
Regards Noman