You can fit a daughter board to the old ecu's which allows remapping. ESL offer this product/service, as do various mappers.

But given it is almost the same price as a proper aftermarket unit which chucks the MAF in the bin, and you are tied to certain individuals to remap, it isnt very good value for money IMO.

Best option on a budget is SimTek, but it is still close to £1k

And I may be, but I'm still right