Hello Eric, welcome to The Realm. As Byron indicates, back in the day the HL510 was a successful racer. Unfortunately for you, it wasn't extremely successful as a production car, and most of the production run has since succumbed to a lack of enthusiasm in the following years. That's recently seen a bit of turn around, with people looking for something a bit vintage and finding available cars such as yours, but you will be dealing with a lack of body and trim parts.
To help you on your path, I strongly recommend you pick up the "How To Keep Your Datsun Alive" service manual. It will be a great help with anything mechanical on your car. That book and some other basic suggestions are mentioned here:
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Within that link also resides links to The 510 Archives. Particular attention should be directed to the back issues of The Dime, Quarterly, because this issue will give you all you need to know about L-series heads:
DQ Volume 12 Issue 2 - Tech How-To: L-series Cylinder Head Guide — Nissan produced many different cylinder heads that will bolt to the L-series block. As these parts become harder to find, it is important to know what you need or what you want before shelling out cash to buy a used piece. DQ lays it all out, including information on unobtainium heads.
The FIA head bolts on to the L block, so no issue there. The problems with that setup include cost (likely a few hundred percent more than you paid for your HL510), finding a complete setup (the head bolts on, but everything that bolts to the head is unique), and driving it on the street (the only manifold made for the FIA head is for 50mm Mikunis...so if you think you can deal with an ancient 2.0L engine running 50mm carbs on the street, this might be for you!).
On the flip side, there's PLENTY you can do with your L20B to hop it up and make decent (but not staggering) power. All depends on your skill and budget. Lots to read up on, including:
DQ Volume 12 Issue 3 - Tech How-To: Cylinder Head Modifications - An inside look at cylinder head modifications. Bruce Godfry details some basic steps to allow the home mechanic to achieve 80% of professional flowing results. It is much more than “port and polish.”
Because when you spend a silly amount of money on a silly, trivial thing that will help you not one jot, you are demonstrating that you have a soul and a heart and that you are the sort of person who has no time for Which? magazine. – Jeremy Clarkson
