Tom , in Vegas here. There are plenty of outlets for your 1200 and A series motors. Nissan Motorsports , whom you can order directly from , supports these cars still. I've raced my 1200 at SCCA events since 1989 , this year I'm switching to vintage and also going to do a Tarmac Rally with it.
For a street car the A14/15 is a good swap. You will get a 12 HP boost over the stock 1200.
I run the A12 due to rules , for your A12 there are a lot of small things you can do to bring the horsepower up.
The electronic ignition from a 210 will fit , a weber 32/36 down draft , header , A14 Valves , clean up the ports in the head and a mild street cam will get you to around 85HP (remember stock is 68). Throw on some sway bars , good shocks and better brake pads and you will have a fun little street car for an additional $900-$1000.
Once you go beyond mild to moderate tuning the maintenance intervals are very short...A12 race engines are about 140 HP but only go 6 hrs between rebuilds.
My racer started life as a $280 daily driver was up to $1800 at the first race weekend and now has about $5000 total invested , not bad for 16 years of fun eh??

Tom

PS I flog my car and generally show it no mercy , these things are tuff