Anne is correct that you cannot contribute to the 401(k) that was established by the C Corp but you can establish a 401(k) for your sole proprietorship and possibly contribute more than to a SEP. The assets that are in the existing 401(k) can be rolled over into the new 401(k).
Depending on the amount of your net income (line 31 on Schedule C) you may be able to contribute more to the 401(k) than to the SEP because in addition to being able to contribute 25% (the employer contribution which is 20% of your net earnings from self-employment) to the 401(k) you can also make contributions that are considered elective deferrals. Your elective deferrals can be up to $16,500 ($22,000 if age 50 or older). However, the maximum contribution you can make combining both your elective deferral and your employer contribution would be $49,000 ($54,000 if age 50 or older).
Another type of plan that you might consider is a SIMPLE IRA which could permit you to contribute more than a SEP if your net income is less than about $74,000 and depending on your age.
In reference to roll over from former C-Corp to sole proprietorship , can we roll over 100% of C-Corp. 401K consisting of both husband and wife into sole proprietorship solo 401K , with my husband of business name ?
If both of you have accounts in the 401(k) plan then you can both roll over to a new 401(k) if you will both be participants in the new plan. If you will not be working for the sole proprietorship then you cannot be a participant in the new plan. However, you could roll your account over to a traditional IRA on a tax-deferred basis.