Hi and welcome to the forum!
I've seen your car before, a long time ago when it wasn't a rally spec car. I always wondered what has happened to car.

Looks good!
Yeah 160hp to 180 is definately possible from 2l also but it takes some cash. Key word is the head work. That's where the power lies.

Get 531 head and take it to some good and known machine shop/guy who knows his business. Tell him what kind of engine you are building and your settled with that. This is the most costly part also, bigger valves, porting etc... are not cheap usually, but good head work is the key to power.
Then some rods which can handle the revs and get all rotating parts lightened and balanced. And also proper bolts for flywheel, it's not a pretty sight when a flywheel comes loose at +7000rpm.
And of course exhaust manifold, intake etc... needs to be sorted. Tubular exhaust manifold and some 45-50 side draft carbs or good programmable injection. I don't know the regulations about engine managment though, lot's of carbs in rally scene, though it could be a reliability thing also.
Though I personally might not put all that work in B200, I would do the cheap mods classicswede presented above and then switch to 2.3l (which can be easilly upgraded to 2.5, many rally cross cars are 2.8 too but that's a lot more costy) because there's not much point putting all that money in B200 block.
About the drivetrain, gearbox isn't the most weakest part, first goes the diff, especially if it doesn't have any kind of locker it goes very fast in hard use.. And as known lockers (except welded diff) are very rare for 300-series.
You can get new gearings and strengthened racing boxes from SSF-Pyörä OY, but those are not cheap.
http://www.ssf-pyora.fi/
Clamps which connect the torque tube on to the gearbox are also one weak point, those might get loose or snap in harder use, but those can be strengthened also.
Some people have also adapted the diff from 240 on to 300-series but that's not so simple job, if you want to keep the box at the back. You need to remove the half shaft tubes and make proper flanges for all the joints etc...
There's one modified diff like that for sale at nettiauto.com but the price is staggering. There's also a suitable Getrag box to go with that diff and also suitable propshaft and Porsche half shafts. Well actually there's also a former Group-A 360 for sale also, same guy who is selling the drivetrain parts..
http://www.nettiauto.com/viewVehicle.php?id_car=820601
http://www.nettiauto.com/viewVehicle.php?id_car=939128
http://www.nettiauto.com/viewVehicle.php?id_car=990564
That car had gearbox in back like the original one, it had Porsche half shafts and M40 clutch. Complete drivetrain package would be 1700€.

Though that kind of drivetrain would be bulletproof.
Damn.. If I would have the cash, I would buy that package my self..

It would be a complete set.

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