seems like you are stuck with "jahaliyat"
There is a guaranteed way for 100% success in repair of your engine, it is to follow the engine repair manual printed by suzuki (the manufacturer).
Similarly there is a guaranteed way of making the engine run a very long life (its mentioned in the owners manual)
in your case, you have a gasket of unknown material and head bolts that are no longer reliable and also dont know how they were torqued down.
The disappearing of water in the engine is mostly by water seeping by into the water pump (waterbody) shaft and seal. The design of the pump shaft and seal allows a slight leak to provide a lubrication and cooling down function for it, Rust/debris/particles eventually destroy the seals and you get a phantom leak that you cannot find. Plain water does not have good lubrication properties like coolant does either.
A very slight loss of coolant level is considered normal - this slight loss is more like 25 ml over a period of months, not daily.
A good repair on your engine is only possible if you follow the steps below - which also is mentioned in the service manual.
1 - buy genuine steel gasket
2 - buy new head bolts set
3 - buy new water pump.
4 - thoroughly clean the entire cooling system of rust/scale/debris etc.
5 - use engineers straight edge to check for flatness of head and block.
6 - use a torque ratchet/torque wrench to tighten the bolts and then do the final stretch just as the manual tells you. - which is after making sure threads are clean in the block and the new bolt, no lube on threads, install washers with a drop of machine oil, tighten to 30 lb/ft starting from centre of engine in criss cross pattern, once done, use a breaker bar and tighten the bolts 60 degrees in the same pattern as above, once done, tighten again another 60 degrees in the same pattern - give the bolts a cool down time of 15 minutes between the last two pulls. This is the way suzuki mentions for M13a (new bolts required)
use good branded fresh coolant/distilled water mixture and fill system to capacity - make sure a good and reliable thermostat valve is in place.
start and purge air from system. pack up and test at normal temperature.
for radiator - buy the aluminium one - it will work, just make sure not to use old style silicated green coolant in it, you need something like toyota red coolant.