The 'case neck' where the crystal fits is supposed to be 28.2mm outside diameter...if genuine spec. Where the trouble begins (assuming the neck is 28.2mm) is with crystals and crystal retainer bezels (inner bezels). Almost every aftmkt number 19 crystal is different, some will fit and some will not. Besides that, some case makers may have a genspec case neck and use an odd spec crystal along with an odd spec inner bezel. This equals trouble as there is no telling who might have a suitable replacement crystal.
I found only one brand of aftmkt crystal that is always the same...GS. Unbranded crystals from supply houses and internet sellers may be the same from batch to batch or not. The catch is GS 19 crystals come in three sizes: PA 462-66 (30.45mm) PA 462-67 (30.48mm) PA 46220A (30.30m), more if you count the 'hi dome' models. You also will need a precision digital caliper, not a HK cheapie. Last but not least...you need a good crystal press with metal dies and develop a 'feel' for the proper fit. After you press the inner bezel down over the crystal (assuming it 'feels' right) take a 10x loupe and a bright light and look closely all the way around the crystal skirt for cracks by looking down through the sidewall of the crystal. If the inner bezel was too tight, the sidewall of the crystal will crack but it may not show without a close look.
After you know what size and brand of crystal will fit your case/inner bezel combo you can go from there. My advice is to find what fits and buy at least three if you plan to keep the watch. Btw, it's not cheap or easy if the first few crystals do not fit so maybe you will be lucky and hit a good combo first or second try. I have maybe 30 assorted number 19 crystals from genuine to Sternkreuz, GS, Best Fit, Clark, ST etc and have tried them all on various projects. I remember on one of my MBK '5513' projects I ended up with an ST/Clark bezel kit (same thing) and a GS crystal (?? on the p/n). The original MBK inner bezel would fit the GS crystal but the rotating bezel was not a good fit on the inner bezel etc, etc so I went to the ST/Clark kit. So far, so good.
I posted a full dose of what to expect because it sounded like you wanted the watch to be water resistant.
Crystals: http://www.ofrei.com/page419.html