I have a long history of developing for weird BASIC systems, and a lot of expertise across over 30 years of IT consulting of using BASIC in various forms to achieve goals (quick integration, ETL, etc). I've written some microcontroller apps before, using various BASICs, C, Assembler and FORTH. I have tinkered with BASIC STAMP in the past, but not recently (I believe I last encountered a STAMP "in the wild" around 2006 or so), so will spend a little time re-acquainting myself with its peculiarities.
I'm based in the US, but I'm confident that we can execute remotely (I've run and participated in many global projects and with global peers on some very complex deliverables). I would rely on you to install and execute any code, aalthough this adds some lag to the dev cycle. I couldn't even begin to build directly, since none of my computers has an RS232 port!
When I've reviewed the documentation and your existing problematic codebase I can provide a better estimate for completion. From the budget I expect it to be less than 10 days, but you have provided insufficient information as yet for me to make a real estimate.
Which device version is being targeted (I hope the latest)?
I'd expect there to be some challenges if your code executes timing-critical tasks... without local hardware those can lead to hard-to-find bugs...
Regardless I look forward to working with you to resolve your challenges.