I'd use straight water for the initial startup. If you gotta open the cooling system a little spilled water is no biggie. Always easy to drain out some plain water & add coolant when everything checks out okey-dokey.
'coolant' does a lot of things & long term it's usually better, but for this short term startup . . .
BTW, a 50/50 mix cools less efficiently than straight water.
You can run cooler with straight water than with a mix. But then you don't have;
- lubricant for the pump.
- corrosion protection.
- lower freeze/ higher boiling points.
- etc.
I'd get a couple of CO2 fire extinguishers. If engine won't stop, discharge into the intake & it will stop it w/no damage.

They some in handy for fires too.
