In days gone past I believe I had an OMP cage in my Fulvia. As has been mentioned there is a helpful mounting (at least one) on the rear parcel shelf for belt(s).
One thing to be aware of is that for competition purposes seats are "lifed". This is a pain for competitors but may well result in plenty of secondhand seats which have been used very occasionally coming onto the market. The same applies to harnesses, with the usual caveats about buying safety related items secondhand.
Actually whilst on the subject the normal HF seats are light but flimsy and I have seen them bend to an alarming degree in impact.....