I have a Ford focus lx 1.6 petrol and it is a st replica can I put an st engine in?

The full car is st exept under the hood, engine, air filter ect. I want an st obviously but don't want to pay the money for one just yet, could I put an st engine in 2l or 2.5l if not what other more powerful engine could I put in? If so how much would it cost and what else would I need to change? Such as brakes and stuff?

Sorry but no one on here is going to have access to the price lists for all of those things and the labour involved as well as the spec of both cars to check what else needs doing to the brakes, suspension, electronics etc.
You should also check out what the insurance would be for such a heavily modified high performance car too.
One thing for certain though is the cost would be thousands and there's a high possibility that it would all cost more than selling the car and just getting an equivalent age ST

You don't say what generation your Focus is, so we can't comment on which ST engine might fit.
Obviously the engine from the same-generation ST will physically fit into the space under the bonnet in your car, but the 5-cyl 2.5 engine from the ST version of the Focus Mk2 will not fit in a Mk1 or Mk3 Focus shell.

You would need to change at a minimum:
Complete engine with all ancillaries (pumps, alternator, starter, intake system, electrical system - effectively everything under the bonnet), transmission, engine mounts, springs (front and rear), dampers (front and rear), brakes (front and rear), instrument pack, engine management system, exhaust system.

Total cost is difficult to assess, but it probably only makes financial sense if you have a written-off ST that is mechanically sound and use your car as a donor body shell into which you fit all the ST parts, and have the time, space, ability, equipment and helpful friends to do all the work yourself. Effectively you would be repairing the ST, not modifying your LX. If you had to buy all the ST parts individually, and then insure a modified car it would cost a lot more than selling the LX and buying a proper ST. A modified car will always be more difficult to insure than the factory-standard equivalent. If you have to pay for someone to do any of the work, or buy any equipment, or rent workshop space, then the costs will be unreasonable.

Sell the LX, buy an ST, and save yourself the time, money and headaches of taking on what is a major project.