Coolant Leak After Engine Swap – Need Advice (2012 Jimny 1.3, Manual)

Seriously - none of us are there, we can only guess. Get a paper towel, tear/cut it into strips and wrap a strip around the "suspect leak points" - it'll either get wet or it won't.

If that fails, and I don't see why it should, but I'm going on what you're seeing & relating to us - if it's a leak that only shows up after the engine has warmed up, what the pro's do is to pressurize the cooling system with a hand pump.

They're not expensive, you connect it in place of the radiator cap, pump it up to 15 psi and look for the leak.

Just be aware that the pipes into the heater core are crimped in place and don't like being disturbed. I had to have mine replaced which is a dashboard out job. It was however fairly easy to locate as it was also leaking into the cabin.