The Bagheera hubs are 4X98 like Fiats, Alfas and Lancias. Some people use "wobble bolts" to mount 4X100 wheels on 4X98 hubs. I like my wheels to stay on and don't recommend such devices. A bit like screw-in wheel studs - many people use them for many years and have no problem with them even though they are an engineering compromise. Knowing my automotive luck the wheels would fall off before I made it around the corner.
The hub-centric diameter of the Matra hubs is 58mm - like an X1/9. Using other Fiat wheels with 59mm will mean that the bolts will be required to centre the wheel which is not ideal - but given the miniscule difference probably acceptable. You would need to be careful to centre the wheel when mounting it.
I am unsure of the original ET (offset) of the Bagheera wheels but could check for you.
The rear of a Bagheera is very generous. For a laugh I put one of my racing wheels on the back - and even at 15X8 (ET +4mm) and Dunlop slicks in 220/565/15 there was plenty of room. It looked FAT!
The front is much more difficult. 5.5" rims are about the max - you MIGHT be able to squeeze to 6.0" rims if you keep a low offset
Recently I dragged out a set of wheels that I use semi-slick tyres on my Fiat racecar. They are Maserati 14X6.5 ET10. As you can see below the 205/65/14's fit EASILY on the rear. The 185/55/14's DON'T go close at the front. Trouble is that I have now decided that I love the look of these and they MUST be made to work - so I have ordered a pair of 14X6's and will modify the guard as needed.