The best upgrade you can do to a Twenty is to upgrade the wheels to alloy and get rid of the anemic brakes... the steel brakes are much better than the alloy ones and can work well with better pads (Kool Stops).

There are a number of long reach BMX brakes that are far better or one can use a drop bolt so that shorter reach brakes can be used.

You can buy a decent alloy front wheel off the shelf although a modern axle is wider and some modifications to the axle or dropouts will be needed and the rear wheel will need to be rebuilt with a better alloy rim.

Just running an alloy wheel on the front makes a world of difference and greatly improves the stopping power.