I am planning to develop (make some minor changes) to asymmetric encryption (i.e. RSA). Any suggestions please?
Well, one suggestion: don't do it. Or, at least, do it for fun, not for actually protecting anything.
It is exceedingly difficult to design a secure cryptographic algorithm. After decades of research, professional cryptographers have found only one method, which is "a lot of scrutiny": an algorithm will be deemed "secure" only after it has been inspected by dozens of other cryptographers for many years, and none of them found anything bad to say about the algorithm. Every detail counts in such inspection, down to the last bit.
Therefore, regardless of what are the changes you envision and how "minor" they look to you, you will not be able to know if the result is secure or not (and, incidentally, the overwhelming majority of "modified algorithms" are not secure).
Even implementing an algorithm, without deviating from the published standards, is difficult, because implementations will most often leak information about the secret values they process (see timing attacks). A basic RSA implementation is little more than some code which handles big integers (and there are good libraries for that), but protection against side-channel leakages is a full-blown active research area, which cannot be summarized in a single Security.SE answer.