Spark plug blown out of engine?

My 1992 previa gave a very loud bang while driving 60mph.. After that the exhaust note changed to very loud.. After that the engine is very shaky when idling.. Blew a sparkplug out?
Any tips how to start solving this? My first thing that comes to mind is to remove the passanger side front seat and clear the floor so i can see what's going on...
When i drove the now tractor sounding vehicle back to my home for 20 km's. the temps didn't go up and it has not lost any oil.. The exhaust shows no visible damage. It's just very very very loud..

It happens.
You're on the right track to remove the passenger seat. Then remove the plastic door sills on that side of the car, and the seat belt where it attaches to the floor. Peel up the carpet and remove the dozen or so 12mm bolts holding the access panel down. Voila! There's the top of the engine, on the right of the van. A couple of 10mm bolts hold a cover over the spark plugs. If one is missing you'll be able to confirm it. If you're lucky you can just screw another in. I hope that's the case.

I think you can be lazy and get away with not removing the floorboard if you know for sure that it's the number 1 to 3 plug... it's only the 4th one that requires floorboard removal

Took the seat and floor out. Was not a big deal. Pretty small job actually. Aaaand Number 2 spark plug was thrown out of the cylinder.. Threads intact.. So i only need spark plug wires and plugs. Gonna change the PCV valve while under there.

Nice! Lucky you!
Yep, good idea to replace the PCV while you've got easy access. You are likely to find that the grommet and hose are as brittle as glass, so you can replace them, too. PCV hose: 12261-76010 ; PVC valve: 12204-15022 ; PCV Grommet: 90480-18001 .
And is your valve cover leaking? How about the distributor O-Ring? Both get hard, crack and drip oil onto the ground or your exhaust mainfold, where it burns off. You will smell it when you get out after a drive. Distributor O-ring: 90099-14091. If you have an older non-supercharged Previa you will have to pay attention to the ignition timing if you take the distributor out.
But as you know now, getting access is not a big deal. You can do these later if you aren't leaking at the moment.

Rethreaded the spark plug hole and put the new threads on top of the spark plug and twisted it in with thread lock.. I got the threads hand tight with your T shaped spark plug tool.. I hope it's tight enough for those steel threads on the aluminium head.. It runs okay.. But now i have another problem.. The heater core has sprouted a leak.. but this time it started to leak from the coolant hose that i have on with 4 clamps.. Apparently the clamps weren't tight enough.. I tightened them with a ratchet and ran the car and floored it.. Doesn't seem to leak anymore but i can't be sure.. I might need to redo the hoses and clamps in the summer.. But for now, the car is not registered or insured for the road so it's sitting in my yard with a foggy windshield.. (the coolant hose sprayed the coolant on the heater core fins and the fan blew the coolant on the glass..)

I'm considering selling the van if i get it sold for the right price. If someone offers me 1500€ i'll sell it in a heartbeat.
Why? Because i don't really need it anymore. I need a pickup truck that can handle offroad so preferably 4x4. Oh and not a toyota. Sorry guys.
But if i can't get it sold in a good price, i'll keep it as a van for long trips with family.