Is it possible to keep battered deep-fried (pork) crunchy?
The secret to crisp sweet and sour pork is mostly how you serve it

Roast, BBQ, grill (whatever you like) the pork so it fully cooked, but on the lighter side of done. Cut into bite sized pieces

If you want to deep fry (not actually required) use a thin and light tempura style batter (flour, baking soda, beer mixed for ten seconds works OK). Drain well. This can be done the day before and keep it in an airtight container, but reheating is as much trouble as fresh deep frying. Remember, when using a thin light batter deep frying only takes a minute

If not deep frying, finish and reheat under the grill (broiler) till it just starts to smoke

Use a hot Chinese style serving platter and poor in all your sweet and sour sauce fresh from the wok. The sauce and the platter should be piping hot, it doesn't matter if the the pork is not that hot

Pile the middle of the platter with freshly shredded raw cabbage and onion. This should make a low flat island in the sauce just breaking the surface of the sauce. You can use wok fried capsicum, onion, carrots etc instead of cabbage (not as good!)

Carefully pile the pork pieces onto the island

For extra zing add more shredded raw onion on top

For a coastal style, and some fresh shredded coconut and/or mango (not to everyone's taste)

Serve immediately with a large serving spoon, so guests can scoop up the sauce and pork all in one go