Converting Access Qrys With Logical Expressions

I'm upsizing an Access database. Got the data converted, working on
the front end, converting queries to views, but having trouble
converting queries that use logical expressions like the following:
SELECT OrderId,
Sum((BackOrderQtyAvailable>0)*-1) AS ReadyBackOrderItems
FROM OrderDetails
INNER JOIN Items
ON (OrderDetails.ClientId = Items.ClientId)
AND (OrderDetails.ItemId = Items.ItemId)
WHERE (NOT (SitesCustomerTypeId = 2
AND ExpressBackorder =True
AND OrderUrgency = 1 ))
GROUP BY OrderId;

Can someone suggest a strategy to achieve the same result, ie
OrderId,ReadBackOrderItems that I can use in further joins?

Thanks in anticipation
Terry Bell