URGENT HELP REQUIRED-- my SANTRO DYING

WHAT ACTUALLY HAPPENED
hey guys, i have no idea what gone wrong in my santro exec 05.. it used to touch 140kmph on cng..anyway, yesterday, my santro while performing great as usual, suddenly broke its power.. it felt like, the car has totally lost power and major sounds start comin up from engine like "KHARR KHARR" and knocking sound on acceleration.. and in idle condition, the engine would shake the whole car terribly.. i have no idea what has happened to it.. i mean, jus minutes ago, i could even do 0-60 in like 9 secs... the car went hot as well and i got know of it on opening radiator, when a HUGE BLAST of Steam n water came out.. anyway, am safe .. surprisingly, the gauge was still in the middle..

IN SEARCH OF SOLUTION
Today, i took it to I-10 hyundai twin city.. they cudnt figure out the prob at first.. later, they found out by connecting a pressure gauge to the slots of plugs that two of em were not building pressure n "CHAKK CHAKK" sound was coming.. they told me its gaskit is leaked..... further they told me that engine is gone weak as well u shud get it overhauled n told me that it'll cost 45k.... it has done 70k kms till now..

MY observation n cause of it
well, i think that it all started because of exhaust n radiator.. i dont know their relation to the prob though but all this might have happened after these recent changes..

1. my rusted silencer broke after hitting a spead breaker so i took it a normal silencer shop where the mechanic fitted a section of pipe which was a bit thick but told me to replace the whole silencer as previously, someone had inserted a very thin section in the silencer.. he said that its building pressure..

2. during all the above story, my temp gauge remained normal and in range but the radiator was fully pressurized and steaming.... it never goes more than half.. may be, its faulty..only once, gauge went to the red part..

more clues..

well, now, the cars makes knocking sound on acceleration.. water is coming out from the exhaust pipe.. engine is make the whole car move badly when idle.. and power is down to zero.. my car hardly touches 50

Solution
guys, am new in town from karachi.. have no personal mechanics and company is asking a fortune... plz advise me someone who is pure professional of santro and can fix my prob in lowest amount... i know fixing it in pindi wud be cheaper but i dint go there cuz there are so many shops n i cant trust random shop.. need ur recommendations... plz, tell me what has gone wrong and estimated cost of getting it fixed permanently in detail..

reply asap plz.. thx