HBFP

Revo mix takes a percentage of the pitch output and feeds it into the rudder output.

In control system terms, this is a feed forward input which increases the anti-torque force of the tail rotor as the torque from the main rotor increases as you pull more pitch on the blades.

Substituting a modern brushless motor for the original brushed one should be fairly straight forward. Depending on your exact model you may not even have to change your 4-in-1.

There's a great hack for the E-Sky HoneyBee which just requires opening the 4-in-1 and tapping the main throttle output from the receiver before it goes into the speed controller. Here's one link showing the procedure: E Sky 4-in-1 brushless mod

Swapping the NimH for LiPo

A simple but really effective mod. This heli is so old that it came with 7 cell 8.4V 650mAh Nickel Metal Hydride battery packs. We can do much better in terms of power to weight nowadays with LiPo packs.

A 2 cell 800mAh LiPo pack is an almost drop-in replacement but the weight is so different that the heli ends up very tail heavy if we put the LiPo underneath the main shaft where the original pack was.