Though flathead engines are not so likely to make a comeback in automotive applications, there are some recent developments in this field targeting the aircraft market mostly because of an upcoming phaseout of leaded AvGas.
Anyway, an engine like the D-Motor LF26 would be a reasonable option if I were looking for a replacement engine to my dad's Impreza...
D-Motor LF26 aircraft engine specifications, D-Motor USA Fort Pierce Florida.