You need to remember that when the 914 was designed, seat belts were still optional equipment on American cars. My parents bought my 914 in 1973, and I still remember riding on the center seat all the way from Oklahoma City to Dallas. Only restraint I had was my Mom's arm across my chest every time Dad moved for the brake pedal!
And yeah, certainly not recommended for adults. Or children. Or wombats.
Now if you absolutely, positively *had* to do this for short trips, here's how I would approach it. I haven't looked at the seat belt mounts in my 'teener since, well, ever, but I imagine they're much like the ones in my wife's '72 super beetle 'vert or my waterpumper VWs - a tapped boss in the body with a bolt and the removable mount. You should be able to find suitable belts to add a third lap belt with little trouble. Try looking in 80's/90's vintage VWs in junkyards. I'd remove the center 'seat' and tray. Then find a car seat big enough for your youngest to belt into and strap it down.
I'd bet you'll have a hard time finding a seat that'll be big enough for your child yet not make you extremely uncomfortable, but you never know until you try.