Honestly, unless you are racing in a dedicated vintage series you should look for something else to play with. Whilst the XX is still mildly competent on the track, it is old and parts are getting harder to come by. The only place you will find spares is Ebay. If parts do eventually show up, with shipping to AU/NZ expect to pay a LOT. I've sometimes waited 6+ months for a XX part to pop up and then had to pay a fortune for it.

The only thing from the 22 that fits the XX is the spur gear. The 22 is metric, the XX imperial.

The first thing you are going to break is the front bulkhead. Luckily, these are being reproduced and are available on Ebay. The next thing will be front arms which are sometimes hard to find (I recently paid just shy of AU$100 delivered for NIP graphite ones). You will want a brace on top of the gearbox as they snap the top mounts off and are again hard to find and expensive.

If you're just getting nostalgic and only want to get the car running I'd just replace the ball cups, give it a tear down, run it carefully around the block and call it done. If you want to actually race with other people (who will crash into you), look for a second hand 22 or similar and put the XX on a shelf.