What would items would I need from a 105 box to use the 105 gears, is it just as easy as swapping the gears onto the x19 main shaft but keeping the x19 final drive?
Edit - I think I need the input shaft (gears machined onto that shaft) and corresponding gears. I final drive will be kept along with the main shaft from the x19
yep you need the input shaft plus the fifth gear on it's end and all the gears from the cluster shaft, the gears are matched pairs, so no mixing and matching is possible (except for a pair of fifth gears that you know run together)
Yes you could reuse the CW and pinion of the X19 5 speed trans, but as I've already said that will be a poor choice for stage rallying, a shorter final drive while it may limit top speed, increases the torque applied to the wheels as gears are "torque multipliers", you want to be 3/4/5 as much as possible, but still have a useable 1/2 for very tight turns and switchbacks which is what makes the 105TC trans preferable to the 100s - you'll have a useable first.
The next thing to note is any transmission that was mated to a dohc engine like a 100s regata or a 105 ritmo will require the input shaft splines to be shortened, as the twin cam input shaft is longer.
And you definitely want to save up for a limited slip differential centre. The standard five speed cast iron diff carrier is the "weak link" in the five speed transmission. The cast iron isn't that strong and has some big holes cast into it, these are the weak spot. The diff carrier will crack and then takes out the trans casing with a big bang. An aftermarket LS differential centre will be machined from a far superior grade of metal and won't have this problem
The gears themselves are not the weak point, anyone that says so really doesn't know what they are talking about. The standard gears use a high tooth count, this is to make them as quiet as possible during operation (and possibly for less frictional losses) A coarser tooth (lower numbers per gear) is stronger than a fine gear tooth (like in a stock X19 trans) so the gear set from a 125/130 ritmo would be stronger than the 105 gear set due to this (but AFAIK and from pictures the 125/130 looks like it still uses the same diff carrier)
Finally for competition use a transmission oil cooler would be a wise investment,
my lemons build thread details how I went about designing a suitable transmission oil cooler setup.
SteveC