Hi! Im new in this forum. I have a Ford Escort RS1600 MK1 1969, 4 speeds gearbox. I get a max speed of 90-100 at 5k rpm, but i want to get another gearbox, a faster one, with 5 speeds. Any suggestion? thx :D

Change diff ratio?

Are you sure your car is a genuine RS1600? I thought RS1600's started production in 1970. Anyway as JVC063 said a change of diff ratio would be easiest, you must have a very short diff to do 5000rpm at 100kph! One of the motorsport options I would guess.  You could calculate once knowing the tyre size, correct revs, GPS verified speed  etc. Or simply pull the diff out and count teeth.
Are you sure your tacho and speedo are reading correct?
A gearbox change to one with 5 spds could also be an option, depending what gearbox you have now it may be easy or hard. For a genuine car I myself would be be reluctant to modify the tunnel though. 
Can you post any pictures of your car, gearbox from underneath, diff etc?
Where are you located?
 
rgds,
SFE

Simple answer NO, escorts had four speed boxes as standard. There are box swaps that are almost compulsory on different builds like the celica five speed is a perfect swap.

Standard gearbox for a genuine RS1600 would be a C/R 2000E box like a twincam had. However if you were in the UK at the time buying new I imagine you could order your build with a 5spd ZF from either AVO or RS dealer complete with the required tunnel mods to suit.  Afterall you could option up to a replica works car. As long as you could pay for it.
A nice BDA warrants a nice C/R box, Celica 5spds are almost as bad as Ford's boxes with too low a 1st gear. If the diff ratio change wasn't sufficient then perhaps a C/R gearset in a Celica T50 with a shortened gear stick might do the trick but tunnel mods might still be needed. 
You can get 5spd versions of the 2000E box but as far as I know they are syncro-less dog boxes.
SFE

From your orig question, ARE YOU TALKING 90-100 KPH OR MPH ??????????
....................not a big problem if MPH and it's a road-going car. If KPH, then all of the other guys info carries more weight.