Firstly, it is important to understand that there are 2 categories of JSPS Postdoc: one for Japanese applicants and another for foreign applicants.
UPDATE: The previous text was incorrect: according to last year's FAQ posted on JSPS site, it says there is only a limit in terms of simultaneous applications by one host researcher, but there is no limit to the number of Postdocs that a researcher can host:
Q2002: If I, as a host researcher, am already hosting a fellow, may I
also apply to host another fellow? A: Yes, you may. There is no limit
placed on the number of individual fellows that a researcher may host.
However, the hosting of multiple fellows should be done in a way that
is not disruptive to the hosting system. As many researchers must be
supported under the program with limited financial resources, limits
are placed on the number of applications that may be submitted at one
time. Please check the Application Guideline in advance.
Nevertheless, this fact does not have a significant effect on the rest of my answer: just because a professor is allowed to receive multiple people does not mean that they will necessarily accept anybody. Some professors may decide that they only have time, resources and patience to accept one single person, or even no one at all.
In fact, I know one case where a professor had already accepted one JSPS applicant, and refused to accept anyone else from a different category, unless the applicant was strongly recommended by some trusted fellow colleague.
To be blunt, you should not have taken so long to reply. At the very least, within the first month you should have sent a short email to apologize for the delay, and give an approximate timeframe for submitting the revisions. By waiting so long, you are at risk of sending a strong message to the professor that you have given up, or that you are not taking the JSPS application very seriously, or that you have moved on to something else. Just because the professor has discussed the proposal with you in the past, this does not give you any excuse to neglect your duty to keep him informed about your situation. In any case, you should urgently contact the professor, give a sincere apology for the delay, and ask if he is still able to accept you as an applicant.
The only person who can confirm whether you can still be accepted as an applicant is the host professor, so you should contact the person as soon as possible.