I did this swap on my 2006 Jimny as the original engine had been run alot and abused by the government department officials to whom it previously belonged to.
In my experience the power difference between the 1.3 and the 1.5 was not alot but it was definitely noticeable.
For the people who truly want a pocket rocket then the M16, M18 or M15-Turbo would be a good choice.
M18 will be relatively easy to find in Pakistan but most of these 1.8 engines will be worn out as these were discontinued in 2007. You'll have to wait to find a good engine or get a beaten up one and get it overhauled. But parts like piston rings, engine gasket, etc will not be available locally because M18 has more girthy pistons hence bigger bores. Secondly you'll need to have the M18 installed with it's original throttle body. M13 and M15 throttle bodies are similar in size to each other but much smaller than the M18 throttle body. If you don't get the M18 throttle body installed with M18 engine, you'll have idling issues and your engine will stutter occasionally because you'll be getting restricted airflow and the engine won't be performing optimally. But for the M18 throttle body to work properly with the M13 ECU, you'll have to swap the M13 Idle control valve to the M18 throttle body.
M18 will also require bigger extractors and exhaust, atleast 2".
1.8 will also put alot of strain on your drive train. You'll be changing your gearbox pretty soon and tires more frequently because of all the fun wheel spins 
It will spin your tires even in 4th gear at 80km/h on wet roads. That's some serious power and will need some caution to drive.
For the M15 engines and your query, these are easier to find locally and possible to find in a good condition. And these will cost you just a little more than the M13 so it's a good option, but only if your original M13 engine is weak and failing. M13 is 87hp, the M15 is 97hp while the M18 is 127hp. Putting these figures here so you can make this swapping out choice youself.
For the VVT to Non VVT question, locally in bahawalpur, they just swap the engine and leave the VVT sensor unplugged. I don't know how it works but it does. They use the original wiring harness which is practically the same thing minus the VVT plug.
For a proper vvt to non vvt conversion, you'll need to swap the camshafts between the two engines.
The physical difference between the vvt and non vvt engine is that the vvt engine will have a bulge on the front. This is where the left camshaft is. This left camshaft is the only difference between the non vvt and vvt engines. For some engines only this one camshaft is needed to be swapped but in my case both shafts had to be changed because there was a difference between the sprockets. Crankshaft's sprocket also had to be changed to keep the timing true.
When you change the camshafts, you'll need the M13s front cover and the valve cover installed on this new engine. In addition to these two covers, the oil sump will also be swapped with the original M13 engine because the M15 sump is of a different shape and might hit the body/chasis during offroading.
You'll also have to figure out a way for the 4x4 vacuum system to be attached with the extractor of the new engine. Some model engines already have an outlet there, some will require a T to be installed.
As of the transmission question, normally people go lower gears with Jimnys to enhance it's offroading capabilities with bigger tires. The only people with higher ratio gears are people into rallies and racing. Don't have much knowledge about higher gears in Jimnys. For higher gearing you'll definitely need to go with a 1.8 engine. After all it's a small utility vehicle and not designed for highways. But still it's pretty good at 100km/h speeds.
