Australian Car Repair Issues? Ask an Auto Mechanic for Answers ASAP
If the 1800 is not worth the trouble, then you are limited to the 1600 MX5 engine.
No other engine will match with the transmission.
Otherwise you are then looking at a conversion, for which an RX7 or RX8 engine/trans is the perfect donor.
Whatever is wrong with yours, it would be cheaper to stay with a 1600, since no changes will need to made to electronics.
Hope this helps.
I hope my advice has been of assistance to you.