Front brakes - piston fell out

[Grahamm]

You have to take the pistons out to check/ replace the seals, so you might as well do that anyway as you're already half way there

But, yes, providing everything is clean, you can just put the piston back in.

And, yes, you'd need to top up the brake fluid (or replace it if it's more than a couple of years old) and bleed the system.