Even if there were the conventional tuning parts sat on a shop shelf somewhere (modified head, high lift cam, sports exhaust, carb etc.) I think the 800cc push rod 2 cylinder motor would struggle to be developed to 100mph. I used to run a Hillman Imp which was 930cc, overhead cam, and with a ported head, twin choke weber etc making about 60hp it would do somewhere close to 100 but certainly not more.
Only way the bis engine will take you to 100 is with a turbo on it, but I can't vouch for whether the innnards of engine and box will survive. If you have the available skills then:
find a good used garret GT12 turbo from a small turbo diesel car
fabricate an exhaust manifold and system to suit.
fabricate intake pipework and install intercooler from small diesel car.
install fuel injection using a megasquirt controller
set it all up and see if it holds together.
i think ti