You can find a good engine from here, most recently a friend of mine bought a 316i engine for an E36 for about 25'000 from Peshawar.
You can go see an engine in Pindi (Gowalmandi) from an 318ti (E36 compact) complete with auto tranny and DOHC 134hp engine. The guy is asking 200'000 for the complete cut piece.
Other modifications I've seen on BMW's are Toyota Supra 6 cylinder engines as they are the best fit for the e30/e36.
There's gearboxes available at Peshawar, a friend recently came back and showed me pictures of a ZF gearbox, don't know how much it costs though.
My recommendation would be to fix the old engine, you can get rings/headgaskets easily from Pakistan and you can get the transmission repaired, just have a mechanic check what the problem is. Sometimes it's a small issue like a hydraulic clutch.
Best workshop would be someone who is known to you. BMW's are well engineered and simple machines to work on, if you want to go to a specialist, spend money and go to Dewan in I-10 or go to G-8 Sarya Chowk, there is a good BMW/Mercedes Mechanic there. Spares are available in Pindi from Zeeshan Auto's (behind bhau mahala).