Gasket easily made from a food packet, pizza box etc. Smear of grease both sides is good. Anything, a bit of the wife's hand cream for example. If the housing is corroded they are available new for reasonable money. Make sure it is the correct one for the B20 which fits the larger diameter hose. Emergency coolant can be anything, bottled drinking water is fine. Collect water from puddles if you have to. Thermostat should be for these engines with the bypass plunger underneath. See pictures here.
https://www.classicvolvoparts.co.uk/...ts_and_Housing