1hr and 45 mins is plenty of time to connect in SIN. All terminal 1, 2 and 3 are connected landside and airside and there are convenient SkyTrains between all 3. Garuda flights usually arrive at the end of Terminal 3, so just take the train to the central atrium and take another train that would bring you to Terminal 1. No security or immigration required. Even if you have checked in luggage, you can ask at the transfer desk who will then assign a staff to collect it for you and re-check it for the next flight.