public class CustomKeyFactory : DefaultKeyFactory
{
    public override BaseKeyViewModel CreateKey(int virtualKey, KeyType keyType = KeyType.Normal, string displayText = null, double width = 1, double height = 1, int alternateVirtualKey = -1, string alternateText = null, bool showSecondaryText = false)
    {
        if (keyType == KeyType.Normal)
        {
            return new CustomRegularKeyViewModel(virtualKey, width, height, showSecondaryText, displayText);
        }
        return base.CreateKey(virtualKey, keyType, displayText, width, height, alternateVirtualKey, alternateText, showSecondaryText);
    }
}

public class CustomRegularKeyViewModel : RegularKeyViewModel
{
    public CustomRegularKeyViewModel(int virtualKey, double keyWidth, double keyHeight, bool showSecondaryText, string displayText = null) : base(virtualKey, keyWidth, keyHeight, showSecondaryText, displayText)
    {
    }

    public override void Update(IKeyUpdateContext context)
    {            
        base.Update(context);
        if (this.VirtualKey >= 48 && this.VirtualKey <= 57) // you may need to extend the if-statement to check for other keys as well
        {
            this.DisplayText = context.IsShiftActive ? this.ShiftText : this.LowerText;
        }
    }
}