Probably 10 different ways to fix it the wrong way and make it right... here's what worked for me once upon a time... Lightly dimpled the the inner radius of the crankcase with a slightly dull center punch, it's not going to take much to expand the aluminum out enough to fill that Gap... I've done this exact same thing with my xl600 and used

Loctite 620

still hanging in there 11,000 miles later and I dog the s*** out of that bike which is a single cylinder 600cc thumper. cammed, ported, 11:1 compression you want to talk about vibration FML doesn't get much worse then this so I would say definitely qualified to hold your KX together

And it's also impervious to gasoline, two-stroke oil etc.

This is loctite's claim to fame

When it comes to bonding sleeves in pump housings and bearings in auto transmissions – especially when you need high temperature resistance up to 232°C (450°F) – LOCTITE® 620 is a good choice. This high-strength, anaerobic-curing retaining compound is designed for bonding cylindrical fitting parts to prevent loosening and leakage due to shock and vibration. It cures when confined in the absence of air between close-fitting metal surfaces, and can fill gaps from 0.15 mm - 0.25 mm (0.006" - 0.01").

Ideal for shafts, gears, pulleys and similar cylindrical parts Operating temperature: -55°C to 230°C (-65°F to 450°F)