IMO, your best choice will be teflon (1st choice) or poly caps. I would try and match the values of the capacitors you want to replace as closely as possible unless you are also planning to rebuild the entire crossover (new resistors & transformers) and plan to change the frequency responses output by the new xover.
You do not need to go hog wild on the caps: virtually ANY capacitor made today, at any price point, is orders of magnitude better than the best caps from even 20 years ago.
After changing the caps, will you Model 19 sound the same? No - hopefully you will find that they sound better. They should sound faster and much more detailed, with better high/low transients and less "bloated" (by comparison).
Good luck