Ignore the nay-sayers. If you want to chase BHP on a 1.6 - go for it. A bit of cheap junkyard tuning is good for a bit of low-price fun.
I have a K-jet dizzi here, if you're interested. This came out of a K-jet fueled Scirocco, so it should be the one with the more aggressive advance.
If it were me I'd hold on and get a PB head on at the same time you do the manifold and downpipe. This'll get the motor breathing much better. Porting work can get quite expensive as you're generally paying for time from an experienced engineer.
You *can* always buy a Dremel-type tool and smooth out some of the bigger lumps on the casting (not too smooth though - you need the inlet charge to tumble). If you test fit the exhaust and inlet manifold gaskets you will see if there's any material sticking out past the gaskets. If there is, cut this back with the dremel, to open the ports out. You can do this both on the head and on the manifolds, so you have matched the ports to the manifolds - this will improve flow as there aren't any 'steps' in the system. It's not rocket science, but it might just help you achieve your 100bhp.
Make sure you've got healthy compression, specifically that your bottom end is sound before going down this route. Also bear in mind that if you dramatically improve flow you might lean the fueling out, so after a batch of mods you may need to re-jet the carb - this is really a job best done on a rolling road - more expense, but that's quite a lot of what turning up the wick on car is all about....
but even the things you have just spoken about all add up to way more than the price of a 2nd hand gti lump.
Theres loads of gti engines around, easily picked up for £150-200.
Dont get me wrong, im not against the chap tuning up the 1.6, after all it can be enjoyable and rewarding.
But from a money point of view, its TOTAL FALSE ECONOMY.
£300,400-500? to tune up the 1.6 with all the parts, rolling road, and not to mention the time to do it all. and all for 90-100bhp
£200 (£300 for a good lump with proven history/milage, or even a whole donor car i.e mot rust failure then sell the scrap shell to get some dosh back) and you have a guranteed 130bhp at least. and could all be done in a weekend!