With less than 100kw in stock form, its hard to justify spending money 'doing up' a 20E.....
The exhaust manifold will be the hardest thing to find (twin cam RB manifold is different), the VL-t (VL Holden turbo) also had an RB30ET so there is another option.... but good luck finding any, the car is rare as hell, let alone finding parts for one of them! Otherwise its a custom made manifold and to do one of the right will set you back big $$$.
You're struggling to find any info because people don't like throwing money away
Gains will be minimal for maximum effort and cost..... there are no mass produced, off the shelf parts.
If you spend an equal amount of money modifying a 20/25DET and a 20E, I know what one will produce more power.