Don't waste money on remap on a naturally aspirated petrol engine. You can only tune it with traditional, old school methods. Start with induction, ram air, 4-2-1 manifold, exhaust straightening, cylinder heads, cams, bore.
As for Jeff's suggestion on the transplant, there are various VAG 1.8T engines. The one I've been in has a BAM which is meant to be 225bhp in the TT, that one has a K04 turbo as well. mk3 GTI 1.8T was 150bhp, IIRC. But it cost quiet a bit more than £6k with a BAM (think double). The cheaper option and I've yet to allocate time and budget is adding a supercharger, the famous Eaton M45 often change hands on ebay. If you're serious about it, then you'll find plenty of references on the internet. For reference, you can supercharge a 1.6/1.8 mk1 MX5 with one of those and the budget is roughly £1000. Price changes depending various factors such as what ECU you choose.