An adjective that would substitute "underfunded" here?

I can’t think of any adjectives that would convey the same sense and meaning as underfunding in the context you’re using.

However capital would be a more appropriate word to use, given your context –

From OED –

capital, adj. and n.2

Real or financial assets possessing a monetary value; the stock with which a company, corporation, or individual enters into business; the total sum of shareholders' contributions in a joint-stock company;

And in phrase use –

capital funding n.

(a) capital raised by a business or company from lenders or shareholders;
(b) funding for capital or fixed assets.

Used in your context –

Tom lacks capital funding to start up a business.

Any adjective meaning poor, prefixed by too is fine.

too poor

too hard-up

too cash-strapped