I prefer to remain silent and low profile most of the time. Maybe it's because a lack of perfect knowledge. However.... idk why but... you sound like you just got ripped off, big time, to the tune of rm550. Go get a refund. It's simple, you see, if the mechanic says the carburetor got problem and replacing it will fix it - and after replacing it, it doesnt fix the problem, means he did a wrong diagnosis, replaced a part that was not actually necessary (based on the fact that it didn't solve anything), and has no business charging you for a solution that did NOT work.

Here's a simple way to determine if you should pay for anything: Did it solve the problem? Yes: Pay the guy. No: No payment necessary. Seems like this mechanic is not good at diagnostics. Many cheap ones arent. I'm not too surprised. You can get back your money if the mechanic made a wrong diagnosis - means you purchased a replacement part UNDER ADVISEMENT FROM THAT MECHANIC based on HIS WRONG DIAGNOSIS. You figure out how to get your RM550 back either directly or via small claims court. NEVER EVER ALLOW ANYONE TO RIP YOU OFF LIKE THAT. and the only way to prevent yourself from getting ripped off like that is by having KNOWLEDGE. A person without knowledge is ALWAYS going to get ripped off, regardless of the situation. What are you going if the replacement carburetor from Japan fails as well? Pay through your nose? Or accept a RM4k-5k engine overhaul suggestion from the same mechanic? Or sell your car since its "having problems"?

Go learn basic engine tuning and diagnosis dude. It's not too difficult. Watch youtube videos on how to tune/repair engine issues. Some suggested highly informative mechanic's channels: scottykilmer, chrisfix, oldskool funk, Ratchets And Wrenches, ericthecarguy and a number of others. You can put in symptoms of your problem in the YouTube search bar and there might just be videos on how to diagnose your specific problem step-by-step covering ALL the potential causes. YOU do the diagnostics, tell the mechanic what you want done. With you having the KNOWLEDGE necessary, nobody can rip you off like that anymore.

TLDR: Get your money back from this "spare part pushing mechanic" and go to a proper high end mechanic that knows how to properly diagnose your issue. If you can spend RM550 for parts, pretty sure you can afford a good, high end mechanic - even if ONLY for diagnostics. Once the diagnostic is made, and say any part REALLY needs to be replaced, then under advisement from the high end mechanic, go replace that part. NOW you can ask your cheap mechanic to do the simple part replacement - since he's "good" at it. Never allow a cheap mechanic to do diagnostics dude, it will always cost you more in the long run.

Cheers.

Edit: Added more specific suggestions based on your info given:
Search in youtube:

Engine Stutter:
1. Ignition timing / engine timing tuning / https://www.youtube.com/watch?v=kgv0wHE895w / https://www.youtube.com/watch?v=kYel4rjHGTQ
2. Carburetor tuning / https://www.youtube.com/watch?v=ZedF7QsnYag
3. Carburetor Cleaning, Replace Engine Air Flow Filter.

Engine Die:
1. Cleaning or replacing PCV valve / https://www.youtube.com/watch?v=7V0q47L2LYI
2. Check valve gaps/ https://www.youtube.com/watch?v=x0H7CP2QLxA

Beyond this, need more info for a better diagnostic, but im really not interested in continuing this because im not a trained mechanic. Cheers.

Edit 2:
Forgot another basic check:
1. Spark plugs need replacement?
2. Check spark plug wiring condition.

Btw for a Iswara, all of the 7 or so suggestion probably cost less than RM100 to pull off. But then again, emphasis added on the fact that there is not enough information yet to make any kind of diagnosis or suggestion. A tip: If you want help with diagnostics from forums, the best way is to usually record a video of the engine sound or engine starting up (since you have engine shutdown problem) in proper, full HD and upload to youtube. only then anyone can have the bare minimum proper info to even start helping you in making a diagnosis or suggestion. The best and fastest way is always a high end mechanic if you need your car fixed really fast (esp if its a daily driver used for work etc). Cheers.