yes you could use a demompression plate.. not the best solution.. but a more financially viable one!
You can get a manifold made up so the manifold fits onto the exhaust ports correctly..
TBH im not sure about the management.. i 'guessing' that the 1800 uses the same management as an XR2i and RST hence you may be able to changejust the ecu and swap a coupple of pins round. however i have NO experience with the RS1800 at all
Phil