Use the standard exhaust manifold and get an ashley group A system. It
runs across the car behind the engine and exits behind the drivers
side rear wheel. It's brilliant because you can't really see it next
to the tyre. It nees to be supported on the engine as it will snap off
if left floating. The XR2 carb bolts on with a small amount of filing.
Fit a luminition. You'll be seeing more power at the wheels than it's
supposed to have at the flywheel by now. Use the gearbox from the 120L
5 speed. It's got the 4 speed diff. In some respects the swing arm
rear end is easier to drive than the later irs, but this won't be an
option for you unless you swap it over. Don't mess with the brakes,
they're already too good at the front. The later panels and glass are
thinner and lighter, but the earlier cars seem to last longer before
the rust gets them. The standard wheels take a 185/60x13 tyre, but
keep your eyes open and 6" rims will appear. The whole car could do
with lowering and stiffening. I have some info somewhere on springs. I
used to navigate in a road rally estelle, and later in an ex-Skoda GB
Challenge car. We developed the road rally car to the point where 16V
Astra's were fair game and it would do 120. This caused a few funny
looks on dual carriage ways!! Especially when the XR3i could ony do
118 and then got passed by a Skoda ;-) I built the rally car engine
and it would rev past 9K without seeming to slow down. Evil.

Have a looksee here.
http://www.skofast.com/
John Shelley spends a lot of time over in CZ, and knows a lot of the
locals. He Also owns John Hauglands ex Ulster Rally Estelle 130LR (the
real Pukka proper job car that Haugy wants to by back and keeps telling
John is "our car").
He is pretty much the UK Skoda godfather, along with The South Coast
boys (Peter the Muppet, and Paul "PSkudman" Wyatt, Merlin Jones and
Graham Boarer), Steve Waterworth with Snotty, and Jay Ivey with his
knowledge gained from 40 odd skodas owned and modified, plus the guys at
TBC who campaigned Ermintrude through a few years, and lots of fun.

Ditch the airbox, fit a bolt on K&N.

Either use the ashley rot away box system, or See John Shelley/South
Coast boys about getting you a replica Group A/B system from CZ, and
mating it to a stainless rear box hung across the back of the car from
the alternator bracket.

The Weber DGV series from a Capri 2 litre, Cortina 2 litre and Escort
1.6 sport or ghia will fit with a little opening of the manifold.  If
you get a DGAV you will need to plumb in the autochoke to the water
system, or if it is a DGEV then it is an electric choke. Manual choke
conversions are available, and can work with the Skoda manual choke
cable/lever.

the 38DGAS off the 3 litre Capri is similar carb, but both barrels open
at the same time instead of progressively, so it is a rev and run car,
rather than a smooth torque monster.

Both carbs will need jetting/dyno work for best emissions.

You can still get K&N bolt on filters for the Webers, or make up an
adaptor plate/airbox and fit a cone filter for cold air feed, but don't
forget carb icing in the winter.

136 engines normally run on unleaded. Look for a 136 rapid, or a 130
rapid with a 136 engine fitted  (either rapid or favorit). Sure fire way
to tell is if the Skoda logo on the casting is painted green it is
unleaded, although most have the paint worn off now.

Early 136's stated no unleaded, later ones said unleaded only, because
Skoda didn't know for sure what would happen with UK unleaded, and
nothing actually changed. 136 engines have alloy heads and hardened
seats. 135 engines have all the same characteristics, but are the low
compression version

The PCD and offset are the same, but you will need to machine a couple
of mm (measure to be sure) from the inner of the spigot ring of the
wheel to fit onto a later i.e 136 rapid front hub. Or fit earlier hubs
to the car and mount the fourpot calipers and onto that. No raised
spigot ring to worry about.

AVO and Spax both do proper Shocks and spring set for the 130/135/136
rapid and estelle models.
GAZ do shocks only. Plus a few of the Sheffield motorsport spring
companies have patterns available for the Estelle and Rapid.

When you lower the back slightly, if you go too far, start to see too
much negative camber eventually, even with the semi trailing arm version
of the suspension.