a 240sx is not/never will be a collector car so don’t worry about it! If you don’t plan on cheaping out pick up a SR block or what not from any importer and build it from ground up. It will make it easier on you in the long run.
Plan b) You can just get an SR and run it till it dies and fix and continue.
You can always talk to Sasha on the boards who raced his S14 with an SR + bigger turbo and took 1st place in CTCC.