Should be able to find an A15 in the papers they pop up every couple of weeks.
Also most engine importers/reconditioners would have one or be able to order one in for you
I'm taking it that you are in Australia seeing as you talked about excels and commodores...
So if you check the classifieds on ozdat.com
*Here*You should be able to find something in there. If not, it gets new classifieds every Wednesday night. So every Thursday morning you can see the new weeks ones.
With the being as fast as an L20 thing: I have raced my friend who has a 2litre 1600 (510) with twin webers and I was equal with him until I changed to fourth gear. he could hold 3rd longer and he also had a few speed so he won.
Although my friends 16 wasn't the best example. That gives you a small idea about comparing it to an L20.
Hope some of this made sense.