No pinto.
Radiator shroud has to be modified to drop a pinto in, because they are too long for a Mk1 engine bay. So even Mk1 RS2000s, from factory had the radiator shroud cut out, so the radiator could sit further forward.
you're a little light on the figures. a 4 door is 840kg.
So, any engine upgrade will be a 1600.
a 1600, with a mild cam, some head work, 32/36 carby, headers, will make a comfortable 70-80kw at the flywheel. which is about double what it currently has. considering i can get the 1300 to rip skids easily, and i can get it to power slide as it is, a 1600 with double the power is going to be more than enough fun.
eventually with a 5 speed box behind it, it would be perfect to drive on the open road.