Tbh, khyber's stock engine, G10, is the best to go along.
I've had my khyber tuned and maintained well and trust me made trips to Kashmir (Till Kel) and with Zero troubles.
The body to perform ratio is what makes it fly like a 1.3, have had a run averaging 140kmph on motorway.
And the maintenance is minimal. An oil change+ filter costs around 1500-2000approx
The fuel average is not so pleasing though, I drive avg 120kmph long and gets around 13 without AC and 11.5 with AC.
Try finding a clean engine at Kabar Market, would cost around 25k if MINT.
Rest your options are other suzuki engines such as g13 (baleno) which to me seems overpowered. Mira or any other 660cc engine with Auto traby which def is far behind in terms of performance or Vitz 1sz/1kr which requires quite some alteration.
Simply, why complicate when simple is the best solution.
Rest is your choice!