Assuming that it's not the clutch?
If the bike goes up & down through the gears okay but stil doesn't go forwards, it could be that the gearbox output shaft (countershaft) has lost its splines that drive the front sprocket? Happened to me twice on a DR600, heard of it happening on a KLR as well.
Easy to check: look at the front sprocket with the engine running whilst in gear, if the shaft's spinning but the sprocket's stationery, spline's have gone.
Fixing it entails a full stripdown as the shaft resides inside the cases. You'll need to load the gears on to a new shaft & budget for a full gasket set etc plus who knows what else on a bike of that age? May be cheaper to buy another engine if you can't do the work yourself?
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