2001 Mazda 323 Astina (Australian) does not accelerate or rev up steep hills
 

Recently bought it from a private sale, however it came with both a roadworthy permit and registration, so in theory it should be in reasonable condition. We'd owned it two days before I went to take it for a drive and went to drive up a really steep section of road near my house. It's a 60km/h zone, so approaching the hill you have to shift from 4th gear back to 3rd to go up the hill without stalling. I did so and expected the car to pick up when I booted the throttle to accelerate up the incline. However, once it started to struggle going up the hill, the revs immediately dropped back to 1-1.5krpm and even with my foot flat to the floor trying to make it go, it dropped back to 40km/h and barely made it up the hill. It drives absolutely fine on flat ground and slight incline, but seems like anything above a 10° incline is where it struggles. As I haven't managed to buy a repair manual for it, I have no idea what the issue could be, however speaking to other people about it i've been told it's potentially a knock sensor in the engine putting the ECU into limp mode as the sensor detects something bad. It could also be a clogged fuel filter or similar fuel issue, or even a clutch problem.
Any help is appreciated, please someone on here be a really smart mechanic lol

Information on the car:
1.6L N/A ZM engine, 1600CC, 5 speed manual transmission, hatchback. Odometer is around the 251k mark, timing belt was recently done.