And they HAVE to be 4-ways, the game wigs out if you swap in 8-ways.
Assault sticks were based on Atari's over-engineered (and very poorly wearing) Atari logo stick. Most Assaults have some pretty messed up sticks and replacements almost never come up for sale and when they do they will usually go for more than what an entire machine will sell for.
Unfortunately the best solution seems to be 4-way sticks from claw grabber machines.