There was never a *canneth; the modals work differently. Only second person singular had a distinct inflection, like thou shalt versus he shall.
You should go to OED and look at each verb for its historical forms. For example, for can, it has:
a. 1st and 3rd sing. can /kæn/, /kən/, /k(ə)n/.
Forms: 1-4 cann, con, conn, 1- can, (4-5 conne, canne; also kan, etc.).
b. 2nd sing. canst /kænst/.
Forms: 1-4 const, 1- canst, (4-5 konst, kanst, 6 canest, 6-7 cannest), northern 3- can, kan.
c. plural can.
Forms: 1-2 cunnon (cunne-), 2-5 cunnen, (4-5 kunnen), 3-5 cunne, 4-5 connen, conne, 4-5 south. kunneþ, conneþ, 3- north. con, can, (kan), 5- can.
Whereas for buy, it has things like:
Forms: 1 bycʒan, -can, (bicʒan), 2-5 buggen, biggen, bugge, bigge, 4 byȝe, 4-5 bygge(n, begge(n), 5 byche. Also 3 biȝen, 3-7 buye, 3-5 bien, 3-6 bie, 4-5 byen, 4-6 bye, by, (4 byi, biy, bii, bij, bi, byȝe, biȝe, byye, 4-5 be, 5 byin, -yn), 5 beye(n, bey, 6-7 buie, 7- buy; 3rd sing. 1 byȝ(e)þ, 2 bihð, 3 bu(e)ð, 4 (Ayenb.) bayþ, buyeþ, 5 bieth.
Imper. 1 byʒe, 3 bu(e), 4 bye, by, pl. 1 bycʒað.
Pa. t. 1-3 bohte, (2-3 bouchte), 3-4 bouhte, 3-5 boȝte, bouȝte, (3 bochte), 4 boȝt, (bohut), 4-5 bouȝt, boght, boughte, (5 bout), 5- bought, Sc. bocht, (6 bowth).
Pa. pple. 1 (ʒe)boht, 2 iboht, 3 boht, 3-4 bohut, (i-, y-)bouȝt, 3-4, 7 boght, 3-5 boȝt, 4 yboht, bowght, (bout), 4-5 boghte, boȝte, (y-)bouȝte, (5 ybouȝht), 5-6 boughte, (6 bouht, bowte, beyght), 5- bought, Sc. bocht.
And where the numbers are referring to century numbers, so 3 means the 1200s. I’m not sure where you would cut it off for what you mean by Early Modern English.