Having it original with the 1st generation hemi would be good as a collector piece, but after all, the hemi isn't rare; just expensive. So I'd put a different engine in it, but not do any modifications that would permanently change or ruin the chassis if later it were to be refitted with an early hemi. That hemi wasn't even too powerful, so you'd do better for performance of the siren with a Chevy 350, which can be had in running condition for $ a few hundred. Make motor mount adaptors (which may already be available from performance stores), and have the drive shaft modified. If I get one, I'd put a Diesel in it. Way more torque than the hemi to get that huge chopper turning, and I think it would be interesting to see how fast it would spin up to speed.
Charles