1,000 liters is 264 US gallons, which also equals 61,000 cu.in.
Without cracking out the exotic design stuff – an aluminum HTP tank:
Operating pressure – 1,000 psi
Proof pressure – 1,700 psi
Burst pressure – 2,200 psi
Should come in around 500 lb
Thin-walled and fiber-wrapped it should be almost half that, 250 lb.
Aluminum is a compatible material for the HTP tank inner liner.
TheTi6Al/4V gas pressure spheres:
Operating pressure – 3,200 psi
Proof pressure – 3,500 psi
Burst pressure – 7,100 psi
Will weigh 55 lb each – use two. They are available.
These will adequately accomodate the pressure and volume required for the proposed HTP load.
So, I estimate, very roughly, the blowdown tankage [HTP and air] to be 350 to 610 lb. Not too heavy, eh?
Add to this valves, burst diaphragms, and pressure regulator.
What does the Cosworht/pump/HTP tank system weigh?