Some difficulties with building a proposed "log" turbo exhaust manifold for the X1/9. In this case I'm referring to the modern "log" style as in the examples by "lanchiahf" and "stingray250", earlier in this thread. However their examples were not for the X1/9, and this makes a difference (as we'll see).
I am not a turbo expert by any means, and have no experience building this particular type manifold. So this is merely some thoughts on what might need to be considered when building one for a X1/9.
If we start with the pre-made "els" (as noted by Simon), using common sizes, the closest to the X's port diameter is 1.25", however the actual ID is closer to 1.38" depending on material and thickness (gauge). Not an ideal size but it could work.
We could use two "Elbow"s (90*) for the ends of the log, and two "T"s for the middle section...as in the prior examples:
Something like this, but as we will see the arrangement will differ for the X.
The "T"s in that size have a total width of 1.75" (according to one manufacturer's chart). That means when you weld two of them together, you end up with a center to center distance of 1.75" for the ports:
.875 X 2 = 1.75
The "Elbow"s are 1.25" from the port center-line to the end of the bend (where it will join the middle sections):
1.25" (elbow) + .875" (one half of a T) = 2.125"
Looking at the X's head / port design, the center two (#2 and #3) are close together, and the outer two (#1 and #4) are spaced out further:
If the distance between #2 and #3 is closer than 1.75" (I have not measured to see), then some material could be trimmed off the "T"s ends to bring them closer.
It appears the gaps between [#1 and #2], and [#3 and #4] are greater than 2.125" (again, I have not measured), so additional material will need to be added to join them at the correct distances apart.
Welding these pieces together and onto a manifold flange (similar shape as the pictured manifold gasket above) completes the "log" portion. Something like this:
Placement of the turbo's flange is the next decision. It can be joined to the log with a beveled section of larger tube:
In most cases it seems the best flow is achieved by locating the turbo in the center of the log, between cylinders #2 and #3. However that might not be ideal on the X1/9, due to the center two ports being so close together. This gives an almost straight shot from ports #2 and #3 into the turbo flange, but a considerably longer run (and around a 90* turn) for ports #1 and #4. I do not know how much this matters.
The turbo could be located off-set to either side of the middle two ports, but this biases the run from the outer two ports. That is how the cast iron manifold for the Uno Turbo is made, mainly to re-position the turbo for packaging reasons on that particular application I believe. I'm not sure if this is any better than the centered layout above. Uno Turbo cast iron manifold:
Another design option is something of a part "new style log" and part "old style log". This might be necessary if the two center ports are too close together to allow use of two "T"s. Here #2 and #3 are simple straight runners hitting the main log head on, which is also a compromise in flow:
This discussion is merely a starting point. Hopefully someone can offer further input about the ideas suggested here; the good and bad points of various options and any fabrication considerations to making a manifold like this.